"To use the washroom" is a polite and common way of saying that you need to go to the restroom or bathroom to relieve yourself or attend to personal hygiene.
Sentences with «to use the washroom»
Use a squirt bottle filled with warm water to clean your bottom after using the washroom. (cindyandjana.com)
Another reason is because I rarely, if ever, use the washroom when out and about. (simplymombailey.com)
Around the world, battles are being waged around the right to do mundane things — such as using the washroom, getting workplace benefits and, yes, dressing in accordance with one's deepest inner identity, freely and without prejudice. (canadianbusiness.com)