Create parallax background scrollable option

I am looking at specifying the scrolling to -100% for Parallax, so that the background stays, as given in the following example for the 1st image. As given in the following example:

https://www.w3schools.com/howto/tryhow_css_parallax_demo.htm

The background image doesn’t stay as the above example , unlike when a parallax region is created via Upfront. Any CSS to fix this?