Ok where you are watching the selected youtube video. On the left hand side they'll be a box called about the video where it has a description of the video, meta tags for search purposes, a URL link and under that there is an embed line of code. Copy and paste that entire line of code and just paste it into the post.
like this classic
<object width="425" height="350"><param name="movie" value="http://www.youtube.com/v/x_8P3D0lNJI"></param><embed src="http://www.youtube.com/v/x_8P3D0lNJI" type="application/x-shockwave-flash" width="425" height="350"></embed></object>