Hi You don’t need extra effort for this if you imported the sample xml demo content. Otherwise you need to follow these steps:

1.Add new post
2.Click to the “Backend Editor” visual composer button (But Visual Composer should be installed and activated)
3. Click to “Add Row” button and add a new row:
4. Click to the “Edit Row” button on the row:
5.Click to “Row Background” tab and define which image, bg, video as bg. Maybe you need to give some padding (ex:15%) for display correctly this video.
6. Done! Video BG is ready, but maybe you want to put something in this row, ex: text or something.

We hope this helps to you. Please let us know if any other questions and thanks for using COSMOS.