Switch to the Full Reply screen, which brings up the necessary toolbar at the top of the reply screen. Next, right click the YouTube address bar, click copy, then go back to the Synthzone reply screen. On the toolbar you'll see a blue icon that appears somewhat like a monitor or video screen. Click that icon (Enter a Media Tag), select YouTube, and paste the YouTube address. Click OK, and the information will be pasted in the body of your message.

Pretty easy,

Gary