How does one 'force' UA's website to show a H,Q,etc fare class when booking on it and wanting to specifically book into that fare class for the purpose of using a GPU?

When I try forcing it, I get "fare class not available" despite it having valid numbers on expert mode for the lufthansa segments. If I don't force it, my "economy flexible" is the U fare. The displayed "economy" fare is W. Can I get something in the middle? Or is the U fare the 'lowest' GPU fare available right now?