Just post the URL. In some forums such as video discussion they'll auto embed. In the rest it'll link back to the original source of the video.
Which section are you posting in? Auto embed is only enabled on a few sections - news, Video discussion, video reviews, I want to say Movies, Video Games and GD as well but can't remember off hand if it is for those.
Ah yes, it's not switched on for Toyark. Afraid there's no way to embed video in that section right now.