How do you post a video from Facebook on here?

 

You can’t.

 

Upload video to YouTube then copy/paste the Share URL link from YouTube.
EDIT: That is if you have the video.

 

I am assuming it is NOT your video. The answer as others mentioned you cannot post it. There is another option is to copy the link if the FB post and put it here. Those of us that have FB account can grab that link and go to that specific post.