Click on the "Share" below the video in YouTube
A link will appear, highlighted - looks something like this:
http://youtu.be/TEoHWLjQoTMCopy it. (right-click and select #copy' or just press Ctrl-C
In your post on the forum, click on the 'youtube' button. The tags will appear, with the cursor in between them.
Right-click and select 'paste', or press Ctrl-V. The link now appears between the tags.
Now delete the
http://youtub.be/ part, so in this case all you would have left between the tags is :
TEoHWLjQoTM
That's it.
It would look like this (but I use brackets instead so you can see):
(youtube)TEoHWLjQoTM(/youtube)