Hi @Madina, thanks for your post.
The background video widget is not yet cms enabled, however you can try other custom code options.
Here is a link to a sample site where I am using Vimeo video as background video using the Vimeo javascript player: https://webflow.com/website/Example-Vimeo-Background-Video
Another option would be to upload your video files to an external server outside of Webflow, then use some html to create a custom code background video, see this post: [Tutorial] How to add a Custom HTML5 Background Video
I hope this helps