Disable scrolling of the background page while lightbox is open

This is currently not an option (I’m trying to find a way that is stable across many different use cases) but you can do this: enable the option to include the jQuery Mousewheel extension script. This will cause mouse scrolling action to be captured and prevent background scrolling at least on devices with a mouse (with scroll wheel) …

And how to connect this extension to this plugin?

There is an option on Settings > Media. Search for “mousewheel” (hit Ctrl+f and start typing mousewheel) on that admin page to find it easily.