Hey, there are two ways to post videos:
1:
In order to see the video as a thumbnail on the main page when you post it, you have to convert it to the .webp format first. There are some hosts like pornhub and redgifs that sometimes work, but I haven't figured out why yet.
I use screentogif (https://www.screentogif.com/) to convert the video to .webp, then I upload it to catbox.moe, copy the link, and put it into the URL field when creating a post. Note that this will remove any sound. I have some more info on settings for screentogif to make it less choppy but it's really getting into the details. Let me know if you want me to post what works best for me.
2:
Just upload an .mp4 to catbox or redgifs and link to it from here. It will keep the sound but the thumbnail won't show as a video.
Right now, this is the best we have. Note that some mobile clients like Sync and Connect do a better job at this then on desktop. If you're concerned about folks' experience, it may be better than what you are seeing on desktop.