First drag your Container widget into the staging area in your builder. If you are wanting to create a new section in your webpage, be sure to drag it in between two current sections and make sure that the thick green line is showing. (For example of this watch video)
Once your Container is placed in the staging area, you will want to select it and navigate over to the content menu on the right hand side of your builder.
Once design is selected in the content menu, you will see the option to add a background video. You can use mp4 or .webm formats for you video. Generally speaking the mp4 format will be the primary format, and the .webm file is optional. Click on the plus icon to select your video from the media library that pops up.
Once the video is in place, you can position the video using the position arrows, and you can set the video to consistently loop. (To view the video playing on the loop, save your changes and view the page live).
Now a container will automatically adjust height to the amount of content within it. So we will add in some space using our space widget to reveal more of the video. To do so, drag and drop your space widget into your container, and then adjust the pixels of space using the height option in the content menu.
Now we see that we have a nice big container that has a video in the background. From here we can add text, images, and any other widget.