ArrayList = Java.type("java.util.ArrayList")
arrayList = new ArrayList()

BBConsumer = Java.extend(Listenable, Consumer, {
    handleEvents: function() { return true},
    accept: function(e) {
        // onBlockBB here
        
    }
})

consumer = new BBConsumer()
registerListener(consumer)
// not registerListener(BBConsumer)
/* I am sure there would be a better name for BBConsumer
And if you want to use it like a anonymous class in Java you could just read the BlockAnimations.js or better,  nashorn document
https://github.com/CzechHek/Core/blob/879f36d7e41ccc7868285b0a808866b360f30822/Scripts/BlockAnimations.js#L35
*/  

Not sure if the type from Java.extend would have other unexpected methods that only takes 1 parameters, you might want to check if the parameter's super class is Event.

Or you could just explicitly specify the accept method in it.
method = consumer.class.getMethod("accept", java.lang.Object.class)

Edit:
Nvm, I forgot that this was scriptAPI, the Consumer here is a Consumer<?>, not Consumer<BlockBBEvent> which means the accepct() takes an java.lang.Object, and it will register a Listener that listens to any event that is an java.lang.Object, which probably means all events.'

Here is what I did

registry = getField(LiquidBounce.eventManager, "registry").get(LiquidBounce.eventManager)
BBs = registry.getOrDefault(BlockBBEvent.class, new ArrayList())
BBs.add(new EventHook(consumer, 
        consumer.class.getMethod("accept", java.lang.Object.class), 
        LiquidBounce.moduleManager.class
                .getDeclaredMethod("onKey", KeyEvent.class)
                .getAnnotation(EventTarget.class)))
registry[BlockBBEvent.class] = BBs

I'll try to simplify it and automize it via Core.

@idk-my-name https://forums.ccbluex.net/topic/94/script-base-core-3-28-simple-yet-powerful-scriptapi-base/30 and here it is

You can use all events if you import Core base, no need to do any of this stuff.
For more info, look at my comment on Core post.

i rember

This is what I came up with without seeing @commandblock2's solution.
I had to reverse engineer a bunch of stuff and then put all together and it worked.

I was wondering what the hell was annotation in Nashorn and it turns out that

print(annotation.class)

literally returns

I'll try to simplify it and automize it via Core.

///api_version=2
(script = registerScript({
    name: "Test",
    authors: ["CzechHek"],
    version: "1.0"
})).import("Core.lib");

module = {
    onClickGuiLoaded: function () {
        listener = new (Java.extend(Listenable, Consumer, { 
            handleEvents: function() { return TestModule.state }, 
            accept: function(e) { 
                print(e)
            }
        }));

        method = getMethod(listener, "accept");
        annotation = getMethod(TestModule, "onUpdate").getDeclaredAnnotations()[0];
        
        AeventManager = new Reflector(LiquidBounce.eventManager)
        invocableEventTargets = AeventManager.registry.getOrDefault(BlockBBEvent.class, new ArrayList());
        invocableEventTargets.add(new EventHook(listener, method, annotation));
        AeventManager.registry[BlockBBEvent.class] = invocableEventTargets;
    }
}

Listenable = Java.type("net.ccbluex.liquidbounce.event.Listenable");
Consumer = Java.type("java.util.function.Consumer");
BlockBBEvent = Java.type("net.ccbluex.liquidbounce.event.BlockBBEvent");
EventHook = Java.type("net.ccbluex.liquidbounce.event.EventHook");
ArrayList = Java.type("java.util.ArrayList")

ArrayList = Java.type("java.util.ArrayList")
arrayList = new ArrayList()

BBConsumer = Java.extend(Listenable, Consumer, {
    handleEvents: function() { return true},
    accept: function(e) {
        // onBlockBB here
        
    }
})

consumer = new BBConsumer()
registerListener(consumer)
// not registerListener(BBConsumer)
/* I am sure there would be a better name for BBConsumer
And if you want to use it like a anonymous class in Java you could just read the BlockAnimations.js or better,  nashorn document
https://github.com/CzechHek/Core/blob/879f36d7e41ccc7868285b0a808866b360f30822/Scripts/BlockAnimations.js#L35
*/  

Not sure if the type from Java.extend would have other unexpected methods that only takes 1 parameters, you might want to check if the parameter's super class is Event.

Or you could just explicitly specify the accept method in it.
method = consumer.class.getMethod("accept", java.lang.Object.class)

Edit:
Nvm, I forgot that this was scriptAPI, the Consumer here is a Consumer<?>, not Consumer<BlockBBEvent> which means the accepct() takes an java.lang.Object, and it will register a Listener that listens to any event that is an java.lang.Object, which probably means all events.'

Here is what I did

registry = getField(LiquidBounce.eventManager, "registry").get(LiquidBounce.eventManager)
BBs = registry.getOrDefault(BlockBBEvent.class, new ArrayList())
BBs.add(new EventHook(consumer, 
        consumer.class.getMethod("accept", java.lang.Object.class), 
        LiquidBounce.moduleManager.class
                .getDeclaredMethod("onKey", KeyEvent.class)
                .getAnnotation(EventTarget.class)))
registry[BlockBBEvent.class] = BBs

the guy named WHO asked

authors: ["xWhitey"],

///api_version=2
(script = registerScript({
    name: "BlockBBEvent",
    authors: ["xWhitey"],
    version: "1.337"
})).import("Core.lib");

module = {
    category: "Core",
    onLoad: function() {
        registerListener(Java.extend((Java.type('net.ccbluex.liquidbounce.event.Listenable')), (Java.type('java.util.function.Consumer')), {
            handleEvents: function() { return true },
            accept: function(e) {
                e instanceof BlockBBEvent && print("Hello from BlockBBEvent!")
            }
        }))
    }
}

function registerListener(listener) {
    for each(method in listener.class.getDeclaredMethods()) {
        if (method.getParameterTypes().length == 1) {
            if (!method.isAccessible())
                method.setAccessible(true);
			
            var eventClass = method.getParameterTypes()[0];

            var adaptedEventManager = new Reflector(LiquidBounce.INSTANCE.eventManager);

            var invokableEventTargets = adaptedEventManager.registry.getOrDefault(eventClass, new ArrayList());
            var onTickMethod = (Java.type('net.ccbluex.liquidbounce.utils.RotationUtils')).class.getDeclaredMethod("onTick", TickEvent.class);
            var annotations = onTickMethod.getDeclaredAnnotations();
            var annotation = null;
            for each(a in annotations) {
                annotation = a;
            }
            invokableEventTargets.add(new EventHook(listener, method, annotation));
            adaptedEventManager.registry.put(eventClass, invokableEventTargets);
        }
    }
}

var TickEvent = Java.type('net.ccbluex.liquidbounce.event.TickEvent'), BlockBBEvent = Java.type('net.ccbluex.liquidbounce.event.BlockBBEvent'), EventHook = Java.type('net.ccbluex.liquidbounce.event.EventHook'), ArrayList = Java.type('java.util.ArrayList');

but it throws an error:

[23:01:10] [Client thread/ERROR] [LiquidBounce]: [ScriptAPI] Exception in script 'BlockBBEvent'!
java.lang.ClassCastException: Cannot cast jdk.internal.dynalink.beans.StaticClass to net.ccbluex.liquidbounce.event.Listenable
	at java.lang.invoke.MethodHandleImpl.newClassCastException(Unknown Source) ~[?:1.8.0_144]
	at java.lang.invoke.MethodHandleImpl.castReference(Unknown Source) ~[?:1.8.0_144]
	at jdk.nashorn.internal.scripts.Script$Recompilation$6736$598A$\^eval\_.registerListener(<eval>:37) ~[?:?]
	at jdk.nashorn.internal.scripts.Script$Recompilation$6733$207$\^eval\_.onLoad(<eval>:11) ~[?:?]
	at jdk.nashorn.internal.scripts.Script$Recompilation$6546$\^eval\_.L:611#L:629(<eval>:631) ~[?:?]
	at jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:628) ~[ScriptFunctionData.class:?]
	at jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:494) ~[ScriptFunction.class:?]
	at jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:393) ~[ScriptRuntime.class:?]
	at jdk.nashorn.api.scripting.ScriptObjectMirror.call(ScriptObjectMirror.java:117) ~[ScriptObjectMirror.class:?]
	at net.ccbluex.liquidbounce.script.Script.callEvent(Script.kt:203) [Script.class:?]
	at net.ccbluex.liquidbounce.script.Script.onEnable(Script.kt:171) [Script.class:?]
	at net.ccbluex.liquidbounce.script.ScriptManager.enableScripts(ScriptManager.kt:53) [ScriptManager.class:?]
	at net.ccbluex.liquidbounce.LiquidBounce.startClient(LiquidBounce.kt:130) [LiquidBounce.class:?]
	at net.minecraft.client.Minecraft.handler$zze000$startGame(Minecraft.java:3428) [bib.class:?]
	at net.minecraft.client.Minecraft.func_71384_a(Minecraft.java:536) [bib.class:?]
	at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:378) [bib.class:?]
	at net.minecraft.client.main.Main.main(SourceFile:123) [Main.class:?]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_144]
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_144]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_144]
	at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_144]
	at net.minecraft.launchwrapper.Launch.launch(Launch.java:135) [game.jar:?]
	at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [game.jar:?]

val eventClass = method.parameterTypes[0] as Class<out Event>
val eventTarget = method.getAnnotation(EventTarget::class.java)

Unfortunately, nashorn/reflection does not have any capability of adding annotations.
method.getAnnotation(EventTarget::class.java)
and we will have to make our own registerListener().

The 3 lines actually does the work is here.

val invokableEventTargets = registry.getOrDefault(eventClass, ArrayList())
invokableEventTargets.add(EventHook(listener, method, eventTarget))
registry[eventClass] = invokableEventTargets

You only need to convert this 3 line to js and it would be fine.
Note the second line
EventHook(listener, method, eventTarget)
the listener is just the listener,
the method is accept acquired using reflection
but for the eventTarget, I borrowed it with reflection from a function in other class that extends Listener already presented in LB.

As for the performance issue, it's true, in my case, putting a print every time it is called makes the frame rate goes from 80(limited) to less than 1.

I might automatize that via Core.

onEnable: function() {
    LiquidBounce.INSTANCE.eventManager.registerListener(listener)
},
onDisable: function() {
    LiquidBounce.INSTANCE.eventManager.unregisterListener(listener)
}
listener = Java.extend(Listenable, Consumer, { 
    handleEvents: function() { return true}, 
    accept: function(e) { 
        e instanceof BlockBBEvent && print("Hello from BlockBBEvent!")
    } 
})