To add a video to the background of a new section, click + add section.
To add a video to the background of an existing section click the section settings cog on the left hand side of the section.
Click on the Background Tab.
Click Add Video
Click a video already uploaded into the Documents and Media area or click Add File to upload a video.
(note: Max file size of a video is 50MB)
Because the video background will not display on mobile devices, you now need to select an image to display for this section when a user views your page on a mobile device. Click +Add Image, then click an image already uploaded into the Documents and Media area or click Add File to upload an image.
Your video will now appear in the background of the section.