This is a business type conversation with W, so I would say it's fine to message her about it. I would say something along these lines,
"Hey! I need your signature for the apartment lease. I will come by at X time today/tomorrow at your office/home"
Keep it straight to the point and don't be passive. Don't ask her when and where. Say when you want it done and if she can't do it, she will let you know. This is a small way to show your assertiveness.
