How do you post a video and have it play?

Copy the link of the video. (Share video from youtube) then where you share pictures there'll be an option to share video link past it there and share👍

You actually have to link to the video outside of SW. Then the person clicks the link and a video plays.

Autoplay? You'd need to code that yourself

I think GIFs work though

Go to the video, copy the Youtube highlighted codes that appear when you click "share".
Go to the furthest left icon on your page, titled "insert image"
Click the title "Insert Video"
Paste the Youtube codes in the box indicated.
Click "Insert Youtube Video"
Then post comment, and you're good.