LR Mobile has an ever-increasing feature set, but it doesn't include watermarks at the moment. You have a couple of choices:
1. Add your watermarks with the desktop version of Lightroom, which is a much bigger part of your subscription;
2. Create a Watermark in Photoshop (also included in your subscription), add it to your CC Library, and use LR Mobile to export Photoshop Mix to add the watermark as a layer.
The second option is cumbersome, but it's an all-mobile workflow (Once you have a watermark saved).
I hope this helps,