Embed a video from "Custom Files" as a banner background

I had been using the Youtube embed function but the video (~11 seconds) was not loading, or loading very slowly. I have an MP4 stored in the custom files area and I would like to embed it as the background banner at the bottom of my site (behind "Ready for the next step?"). I can also upload it as a WebM if that is more widely compatible.

What is the procedure and code to do add this?

Thanks so much!

Site: www.bigskyeducation.org

Stored mp4: https://static1.squarespace.com/static/5c5ca6fe755be251ea169278/t/5d093f1213b5990001966514/1560887083861/Starling+Clip+Medium.mp4

Poster image: https://static1.squarespace.com/static/5c5ca6fe755be251ea169278/t/5d58283c4809950001459ea1/1566058558276/clouds-contrails-dawn-65865.jpg

Chrome inspector results for the relevant block:

div class="Index-page-content sqs-alternate-block-style-container" id="yui317211566060544427701"