すみません、トイレを___もいいですか。
Q. Which verb is used to ask to use someone's bathroom?
Translation
Excuse me, may I use your bathroom?
Explanation
From the asker's viewpoint you borrow (借りる) the bathroom: トイレを借りてもいいですか. 貸す is what the owner does (lend). Note Japanese idiomatically 'borrows' the bathroom rather than 'using' it.