1. When you start a new thread or make a reply in an existing thread using the "reply" option (not "quick reply"), above the section where you type the message, you will see icon buttons of different tags that can be added into your message. One is an icon with the a movie clipboard.
2. When you click on the icon, it will pop up a box that asks you for the URL of the video you wish to post.
3. Click Insert Video.
4. There are several supported video sites and they are listed on the popup box.