It really is simple.
Take your clothes off. They will cover you with a towel if it’s a problem
If they ask you to wear underwear, just say you don’t or can’t.
You can always tell them you understand that there are no sexual services, you arent here for that, but its impossible to massage glutes with underpants on pulling them up or down, just has elastic in the wrong spots.
Shouldnt be a drama as long as you dont start looking to get tossed off.