Using Tweet & Get It with Membership plugin

I’m trying to use Tweet & Get It (T&GI) in conjunction with the Membership plugin. In order for T&GI to work, I need a URL to send the user to after they submit; the page I want them to arrive at will offer a 30 day free trial which is *not* offered to site users as a whole (it is exclusive to those who participate in the promo). It does not have to be secure, per se, just not offered on the main registration page. So my question is how do I setup a subscription which is only available to those users who know the sign-up URL?

I am open to using another free 3rd party plugin or Premium WPMUdev Plugin if needed. The end justifies the means. Thanks! : )