As it turns out, I don’t have to do this. That’s because my membership plugin allows me to enter a different GetResponse campaign for each membership level. (My membership plugin allows multiple levels. For my first membership site I’m using only two levels – free [which is required] and paid.) Because the plugin handles this the buyer is automatically subscribed to the campaign I select for that level in the plugin. The “login” form we’re talking about here handles this functionality. Therefore, because my plugin handles this in the manner I’ve described, I won’t require the “login” form. All of that said, here’s how I was gonna handle it. So maybe this will give you an idea.
In PayPal I setup the order thank you page to display the page that you just tested for me. If my plugin hadn’t handled the functionality “automatically,” my plan was to instead create the “login/opt-in” form we’re talking about and use that as the order thank you page in PayPal. Then, once the buyer pressed the OK button on the opt-in form, the button would call an order thank you page similar to what you tested.