May help some people but indeed is really easy
Youtube video:
Lets choose a random video for example this one <!-- m --><a class="postlink" href="http://youtu.be/cYNdUM2gRsg">http://youtu.be/cYNdUM2gRsg</a><!-- m -->
Now you go to he video and click in the box which says Share and u tick the Long Link part and then u just copy that link
Now to post it on the forums you click on the youtube icon or you just simply write "[youtube]youryoutubelinkgoeshere[/youtube]" without the "" and that is it ;)
[youtube]http://www.youtube.com/watch?v=cYNdUM2gRsg[/youtube]