je dois aller aux toilettes. (I must go to the bathroom) j'ai envie. (I need to {pee}) j'aimerais utiliser les toilettes (I would like to use the bathroom. this turn of phrase is close to what you said but it's not common. I was trying to second guess when and why you'd say "I want to use the bathroom" which is a bit too direct in French. If it was because there was a attendant cleaning them, you could say that to him or her as a way to say sorry to interrupt your work, could you exit the room for few minute) Marc's anwers are good but they only mean Can I use the bathroom?

Puis-je utiliser la salle de bain? Est-ce que je peux utiliser la toilette?