background video full page header

Hi Tom,
I have spent hours trying to do a full-page video header.
I have checked out all your support pages followed.
https://docs.generatepress.com/article/page-hero-background-video/
Tried all sorts of CSS etc.
I can't get it to work, I'm working on a flywheel with a new install the only plugins are GP Premium and Simple CSS.

I have made a number of sites with Generatepress its and amazing theme. But I can't believe I'm the only person who is having this problem. the page https://docs.generatepress.com/article/page-hero-background-video/ is helpful although it does not tell you the settings for the header element, I have tried them all.

Here is my content in the content box of the header element.
The CSS is as the page says.

<video loop muted autoplay poster="http://gptest.local/wp-content/uploads/2020/03/cropped-jan-folwarczny-1177863-unsplash-1.jpg" class="background-video">
<source src="https://www.youtube.com/embed/rtEkZ5En-Xc" type="video/mp4">
</video>

Can you please advise.
Vajra