New to 5.8.x, if an event does not exist in the current list of events it is not possible to trigger the event based on its Java class name.
To set up a listener based on Java class name:
Go to Administration → Manage Apps → Power Apps Config → Power Scripts → Listeners
Select the script you wish to trigger in the “Choose the script” field
Under the selected event, choose Class Specific - Custom Event
Enter the Java class name in the “Specify class name” field