Upload the video on imgur.com.
Once this is done right click on the video, select “open video in a new tab”. It will create a page with just the video file as present in imgur servers.
Copy the url of this new tab (it will be in the format " i.imgur.com/IDofvideo.mp4 " )
Paste this link into the post on the forum.
The forum will integrate the video by itself.
In this case, the video was uploaded at https:// imgur. com/lmw8jHH
But the link pasted into the post is https:// i.imgur. com/lmw8jHH.mp4
(both sans the added space)