There is no any direct approcach to watermark your videos in .net but you can try any third party component. Media Manager Pro at
http://www.all4dotnet.com/ is a good one to go with.
User2022321183 posted
Once I've done watermarking with VidLogo, but I'm not sure if they still offer a trial. Anyway, it turned out to be a nice program, maybe it will be helpful for you)