Go to the YouTube page where you published your video and from the address field at the top of your browser, copy everything between the "v=" and the first & (ampersand) symbol. (Highlighted in red below)
Paste the section you copied where you like into your forum posting, highlight it, then click the "youtube" button above the text field. It should end up looking like this without the underlines:
It should display like this when you preview or save your post.