If you press the "Go Advanced" button instead of posting you'll get a Youtube-icon in the toolbar above the message window. Just click that and add the last part of the link.
"https://www.youtube.com/watch?v=EUS-u7p-Zow" --> Only "EUS-u7p-Zow"
Or you could just write "[@YOUTUBE]EUS-u7p-Zow[/@YOUTUBE]" without the @'s.