Instead of just having the video link handled by Discourse when it bakes the post, you can insert the video HTML element and add the autoplay attribute:
<video width="100%" height="100%" preload="metadata" controls="" autoplay>
<source src="https://www.simscale.com/forum/uploads/default/original/3X/d/6/d6fab77fb56eeb98ef660d682f344dba4bbb66ce.mp4">
<a href="https://www.simscale.com/forum/uploads/default/original/3X/d/6/d6fab77fb56eeb98ef660d682f344dba4bbb66ce.mp4">/forum/uploads/default/original/3X/d/6/d6fab77fb56eeb98ef660d682f344dba4bbb66ce.mp4</a>
</video>
Since the video has no sound, it will autoplay. If it had sound, you would have to add the muted attribute as well.
More info:
You can see what HTML tags are allowed in posts here: https://github.com/discourse/discourse/blob/main/app/assets/javascripts/pretty-text/addon/allow-lister.js#L115