What you have to do is find a video. When you find the video that you want, you have to copy its URL (Look below at image if you don't know what that is). When that's done, go to the forum you want. When writing a new post, three square buttons should appear at the bottom: one is a smile, one is a camera and the other is a video camera. Click the video camera, and this rectangular box should appear. This is where your videos URL comes in. Paste the URL inside the box (It usually doesn't read it if you just paste it, but hitting space afterwards usually does the trick). A picture of the video should appear below. If words can't do the trick, I made a small tutorial to help you out (Click the picture for a better resolution/to make it bigger).
