The code above requires a direct link to a video file. I know Vimeo PRO supports this, though you’d have to research what levels of their service support this feature. 
To do it in Webflow, create a separate page, create a background video element, upload your video, and publish the site (.webflow.io works fine). Visit that page on the published site, right click the video, and inspect. You should be able to drill down in the code to the specific URLs above (WEBM, MP4, and the background image/poster). 