I have a short mp4 clip and I cannot find the right html coding to insert it into my web page

If the video is sitting on YouTube, Vimeo or some other such site, then use the element 'Video Platform'. If you are uploading it, or if it is uploaded to somewhere already, then 'HTML 5 Video' should be used. There is also an erlement for background videos, 'Background video Container' or 'Background Full screen container', if that is what you want.