Sending data via socket to plugin


I'm trying to send data via the Octoprint Javascript Client Library to a custom plugin,
but I'm not sure how to do that.
My guess was to either use OctoPrint.socket.sendMessage with the new custom events,
so for example in the plugin itself:

def register_custom_events(self, *args, **kwargs):
        return ["selectExtruder"]

and then using OctoPrint.socket.sendMessage("", "ON")

and reacting on the plugin side with

self.onDataUpdaterPluginMessage = function(plugin, data) {...}

This did not work, no message was sent to onDataUpdaterPluginMessage at all.

My other guess was almost the same, but not using register_custom_events and instead just send it via
OctoPrint.socket.sendMessage("mypluginname", {type: "light", "data": "ON"}) using the same name as if I was sending data from the plugin itself like self._plugin_manager.send_plugin_message(self._identifier, dict(type=myType, data=myData))

Though that did not register any message sent either.
Does someone has an idea how I can achieve this?
Or would it be possible to include in OctoPrint itself so I can send data on the custom event channel from the JS Library to the plugin?