would like to search just within groups which have "movies" in the title
I assumed you meant "watch lists" and not "custom list". Custom lists are something you feed by hand. Say you load a group or you use search, you can send some of the results to a custom list.
If you really mean "custom list" there's no much point to doing what I suggested.
So, maybe I don't understand what you're trying to do. If you want the search engine to just return results from your newly created list of groups, you just need to select this topic before you search.