At the bottom of the page, to the right of the "submit message" button is some blue-colored text that says "Make a link ('blue clicky')" -- click on that. It will open a page in a new tab. In the first line of the form (to the right of the words "Link URL" copy and paste your link from YouTube. In the second line, paste what words you want the link to say (for example, if the video is of the song "We Don't Know Why" you would put the link to that song in the first box and the words We Don't Know Why in the second box). Once you're done, click "create link" and it will give you something that looks like this: