See openhab/openhab-core#5041 and docs at https://next.openhab.org/docs/developer/utils/events.html#the-core-events
Basically just add &source=org.openhab.android to the query string (or regenerate the openapi wrapper after that PR is merged, if you use that). If you can think of something useful for the "actor" (username or device name), append that after a $.