A direct link to the MP4 in the URL field IMO, however videos in general typically aren't well supported by all the client apps.
The ex-reddit apps like Eternity (FOSS), and Sync (proprietary) IME are generally good at handling multimedia files like mp4, webm, webp and gifs on Lemmy
For users on other apps missing support, something like ImgurViewer from F-Droid works great - it will intercept the mp4 links and open a video player overlay (yes, even for stuff posted outside of Imgur)
Is it already online somewhere? If so, you can post the direct link to the media and it usually comes in full size. If it's a video you took, you can upload it though the post duologue in boost at least, there's a gallery option next to the link field.