The key step for you is to create the event in your Soundreef profile, in time and with a complete and correct setlist (i.e. with all the songs you'll play during the performance). There's only one more thing to do in these cases, which is to fill in the SIAE digital borderò.
Once you've successfully created the event and submitted your setlist, the organizer will be notified and informed that they must apply for both a LEA license and a SIAE permit at no additional cost. Copyright will be shared on a pro-rata basis, so don't forget to fill in the SIAE digital borderò in addition to the Soundreef setlist.