"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. Full definition
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)