First- To do this all you need to do is go to the video you want and copy the url(Ctrl C),like for this video it is https://www.youtube.com/watch?v=WSeNSzJ2-Jw&ob=av2e.
Second-After you have done this come back to the site and go to post a topic.
Third- In the top right of the tool bar you see a video clip looking thing. Click it!
Fourth-Past the url into this bar that pops up(Ctrl V)
Five-Your done write whatever you want preview you it if you want and post!
PS. If you want it centered simply highlight the text and click on the centered lines above in the toolbar!