Normally for full events I might show up with generics to see if there is a spot available from a no-show. Is that still possible this year? I didn't see anything in the Events Tome of Knowledge, and I don't see generics as an option for the online format.
The GM can sell you a ticket from their part of the App. on the spot (as it were). They have a "Issue Ticket" button on their event display. They should know about it (it's in their EO/GM FAQ), but you can point it out to them if they've forgotten.
Their instructions say (in part):
Gen Con, LLC.