The forums use html coding, so if you know how to do that in html, you could.
I don't suggest this though, may not be user friendly to those on dialup. It's best that you just share the link with everybody than trying to embed it.
Only the HTML tags on the right of the textarea are available. Since you need other tags to embed a video, this isn't possible. However, you can put a link to the original video site into your posts (e.g. to http://www.youtube.com/watch?v=...).
