You could add it in during post processing if you felt like it. I've always found watermarks to be an annoyance but, they're tolerable if small and out of the way.
Here's some basic config info:
Image1.jpg
Image2.jpg
Image3.jpg
Click the plus to add a capture source. Right click that, properties, and then choose either fullscreen or specific window as you need to. If you have a HW encoding option with your video card you'll want to use that. I'd suggest setting hotkeys for start/stop recording if you haven't, ie. F11/12.