Find a quiet public toilet stall. Go in there, close the door but don't lock it.
Keep only your panties, tie your ankles and sit on the toilet backwards. If someone enters you can reach for the doorknob to hold it but you cannot turn around.
Since pain makes you horny, spank yourself as loudly as you can. You can stop only when you can feel your underwear is really wet.
Once this is done, untie yourself, take off your panties. Open the door and throw away your wet panties as far as you can outside.
If this is safe, go naked to take it back and go get dressed.
If you think you might be exposed, get dressed before getting it back. You cannot put it back on afterwards then, but you must get it back.
I hope you'll enjoy that
