I've never done this for video files before but I have for mp3 files. You can get access to those frames like this:
TagLib.File file = TagLib.File.Create(mp3FileName);
file.Tag.Title = "some title"; // you've got this
TagLib.Id3v2.Tag tag = (TagLib.Id3v2.Tag)file.GetTag(TagTypes.Id3v2);
tag.SetTextFrame("WOAR", "some url"); // WOAR = Official artist/performer webpage
file.Save();
You can find a list of the text frame identifiers at Wikipedia: ID3v2 Frame Specification (Version 2.3)
I don't know if video files give you the same range of frames that ID3 does, though notice that Wikipedia also says (Implementation in non-mp3s and alternatives)
MP4 also allows the embedding of an ID3 tag, and this is widely supported.
So I would guess this also works for mp4 files like you're trying.