public class ExampleMod implements ModInitializer {

    [...]
    public static final ScreenHandlerType<BoxScreenHandler> BOX_SCREEN_HANDLER = new ExtendedScreenHandlerType<>(BoxScreenHandler::new);

    static {
        [...]
       
        //we now use registerExtended as our screenHandler now accepts a packetByteBuf in its Constructor
        BOX_SCREEN_HANDLER = Registry.register(Registries.SCREEN_HANDLER, new Identifier("mymod", "box"), BOX);
    }

    @Override
    public void onInitialize() {

    }
}