1. Find the video you want on youtube, vimeo, or whatever and copy the embed code (seen on you tube on the right hand side)
2. On the new thread look above the text box, underneath the "Message:" and there's a cursive "F"
3. Click that and a new box will pop up. Paste the embed code into that box.
4. Click embed and you will see it on your text box.
Done.