Yes, you can add a watermark to your video.
There are a couple of things people mean when they say watermark.
If you want an overlay added to your video in the player, we have a few tutorials here:
You can also burn a watermark into your video. The difference is that it is permanent and can not be changed.
In this case, you can use this tutorial: https://api.video/blog/tutorials/how-to-add-watermarks-to-your-videos.