How to read properties of custom events caught with JxBrowser

Question

I'm working on a project where we have a Java application that is opening a Chromium popup window using JxBrowser. When the user performs an action on the UI, I want that to send an event back to the Java application with some data. On the UI side I have a method publishing a custom event:

public sendUpdateEvent(): void {
    const event: CustomEvent = new CustomEvent('update', {
      detail: 'helloworld',
    });
    document.dispatchEvent(event);
  }

And on the Java side we have set up an event listener using JxBrowser functionality.

browser.navigation().on(FrameLoadFinished.class, event -> {
    Frame frame = event.frame();
    frame.document().ifPresent(document -> {
        EventType eventType = EventType.of("update");
        document.addEventListener(eventType, myEvent -> {
            System.out.println(myEvent.type());
        }, false);

This has worked, we've been able to capture the event on the Java side, but the Event interface doesn't seem to give us a way to get the contents of the event? Does anyone know how you could actually read the properties of a custom event like this?