The video file does not need to be hosted by YouTube or Vimeo, you can directly upload from your device or from Dropbox.
The audio component of any background video will not be included as many browsers will not play video automatically with audio included.
The limit of 15MB has been set to ensure that the video loads quickly across all devices and network times (WIFI, Cellular data etc.)
See our guide here for ideas on trimming video that is too large or not optimized.
Create video Title Page

Create a Custom Page > Title Page (see our guide here if you haven't made a Custom Page before)
Select the second template from top-left with the video thumbnail playing
Another video template with Title, Description and an Entry Button!
Video Title Page editor overview

Notification of video background
Tap the background to bring up options 3 & 4.
Add Background Video
Customize Design settings
Text link that needs to be set to link into a page
Unconnected Links warning displayed
Save Changes
Upload video file

Drag & Drop a video file 15MB or under or tap 'Upload Video'
Import from Dropbox
Reminder video file must be 15MB or under
Video upload progress

Video filename
Uploading size and status
Progress and time remaining indicator
Save Changes Warning

Save Changes without linking shows Warning
'Go Back and Connect Links', recommended
'Save without Links' not recommended!
Select and edit text

Replace text with your own and select all text
Tap 'Link'
Create Link to Page

Select 'Page'
Use 'Choose a page' dropdown
Select Page
Save Changes

The orange warning panel at the bottom will no longer appear
Link has been set
Save Changes
Customize Design

Tap 'Customize Design'
Customize Title Page panel

Changes made to are specific to this Title Page (you can have multiple Title Pages within a Format site used for different purposes.
Changes that you make in the left panel are reflected in the in-app preview
Preview in Live Site

View your live site across a range of devices and screen sizes to ensure you are happy with the way video is displaying.
Make any changes you feel necessary.
Auto-Play not Auto-Playing?
If a web browser's preferences have been set to disable the auto-playing of videos (Never Auto-Play) then the video will not auto-play and will appear as a fixed image.
The default 'Stop Media with Sound' would still allow the video to play.
Each browsers settings may be different but the screenshot below is for macOS Safari.

macOS Safari > Preferences > Websites tab
Tap 'Auto-Play
When visiting other websites, select 'Allow All Auto-Play'
Also, try testing in another browser or device if you are not seeing the video auto-play.