I have seen this question asked a couple of times recently, so here is a step-by-step on how to do it:
1) Go to the Youtube video desired, and while it is playing, highlight the URL text at the top of the page (see attached screen shot).
2) Right click on the highlighted text and select "copy".
3) Go to the forum thread where you want to post the link, type whatever text (if any) that you want to accompany the link.
4) Hit "enter" or "return" once or twice to provide some space between your text and the link.
5) Right click at the beginning of the empty line that the cursor is at, and select "Paste".
6) The URL text that you copied before should appear. Enter your username and password in the bottom boxes, and select "Preview/Post Message" like you normally do for a new post.
7) If the preview looks good, select "Post this Message" like you usually do. Your new post will appear with the link embedded in it.