That’s one of the first questions I hear when I tell a boy he’s going into Chastity.  

“But…but… how do I go Peeeeeeee?” 
(there’s always some whining). 

I tell them “You’ll figure it out. It’s part of the training to be a good obedient boy“.

I also like reminding them later while in bondage that “You have sit down in public when you go to piss now, don’t you? Just like a Bitch”.