You don't. The software doesn't save anything about Festivals or Events. It gets that information from the thehendonmob.com. It was my understanding that if you submit a tournament with a custom Event or Festival, that new Event or Festival would be added to the database on thehendonmob.com, and thus be available for selection when you next submit a tournament.
I could conceivably add the ability for it to remember your custom Event or Festival name so you don't have to retype it each time, but that's really just working around an issue on their end.