すみません、お手洗いを貸していただけますか？ — Excuse me, could I use your restroom? (Literally: 'Could you lend me your restroom?') (Convenience · Store, Formal — to staff, JLPT N5)

Gesture & etiquette

Slight bow, hands together in a polite asking gesture, or pointing to self with urgency.

Do not insist or get angry if told 'no.' Understand that most convenience stores do not have public restrooms. Look for public restrooms in train stations, department stores, or parks.