Start jerking your cock while the mixture of the tastes of asshole and piss remain in your mouth. Try to reach an edge while your bladder remains full. Fight off the desire to pee some more while your cock gets rock hard by the combination of the need to pee and the humiliation you have just endured.
That's it slut, stroke the cock. Get to the verge of coming, but stop just before your cock is about to explode.