Determine parent group from Previous Page parameters

Hi, Probably been answered and I can’t find it. How do I set the parent group of a group that opens on a new page from the URL parameters of the previous page. So image 1st Page has a selection of topics, A, B, C etc. 2nd Page has a group the parent of which needs to be determined (and all the other content pulled from that specific thing) from the URL parameters sent through from 1st Page. What do I need to allocate to the top group so that I have a data source option that can pull from that particular specified Thing (A, B or C) from the previous page. I hope the question makes sense. Thanks.