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How to stop background scroll in a simulated lightbox experience

I am new to web design and webflow. Just want to start off by saying this tool is awesome and I have found it incredibly useful, especially being a rookie who has used it only for a couple of months now.

I am trying to create a lightbox type experience where a form needs to be filled in the lightbox by the user. To understand the behavior, you can click on the signup button in webflow forum and check the popup lightbox.

I created the lightbox using a div block with appropriate form fields inside. However, when I now view it in preview mode and scroll down, the background content scrolls while the contents inside lightbox stays fixed. I want the exact opposite behavior in that I want the background content to stay fixed while the contents in the lightbox should scroll down. Can you let me know if the setting to accomplish this behavior? Thanks.