How to make banner full screen over header but go back to normal upon scrolling

[7IGWT]

Hi, I would like to make my banner which is currently a GIF (i can change this to a video if necessary) to become full screen upon entering making the header transparent aswell but go back to normal as soon as scroll is activated.  

I want it exactly like this: https://rstheplug.co.uk/ 

As you can see in the above link, the video automatically becomes full screen making the header transparent but goes back to normal upon scroll. 

Here is my site: https://7igwt.com/