Here's the simplest test I've tried and it won't work.

I have a test form with a single dropdown box on it with 3 choices in it. I create a new form and insert a lookup field. On the Create Lookup Relation dialog, I choose the test app, the test form, but the dropdown from the first form is not available as a choice in the field column. 

Is this not possible or am I doing it wrong? In my live app, I need to be able to select some data on the second form from previously selected data on the first form (like of a filter, or query in Microsoft Access).