Would you mind I leave for a moment? I need to go somewhere. 您介意我离开一会儿吗？我需要去趟洗手间。

Go ahead. 请便！

I want to wash my hands. 我想去趟洗手间

Excuse me. I want to wash my hands.  对不起，我想去趟洗手间。

As you like. 请便！