Answer
At IFS Cloud/Apps10 Update 10 and earlier releases, Custom Event functionality wasn't available directly within Aurena at all — it was introduced starting with the 21R1 release. On any release before that, Custom Events (and some other Solution Manager-style configuration) still needed to be done through Enterprise Explorer even in an otherwise Aurena-based deployment. Check your current release's documentation to confirm whether Custom Events are available directly in Aurena on your specific track.