Screenpop, filter report on custom object

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Hi

I'm trying to use screen pop to load a report and filter on a run-time selectable filter which is a list of custom objects. So the url I'm loading looks something like this, http://localhost:fakeportnum/screenpop?t=report&id=12345&customobjectname.c$sub_group=Productname

I've obscured some of my query string here, but it's basically the gist of what I am doing. I worry that the dollar sign or the dot before the c$ or even the underscore was causing issues, so I've tried replacing those with their URL encoded counterparts, but none of that worked either.