Use Gravity Forms for initial registration, then send to Pro Sites to upgrade

I’m want to use Gravity Forms for the initial user registration/site creation, just to replace the default wp-signup form, but then after creating the customer’s site, send them to Pro Sites to offer the upgrade.

I’ve created a registration form with Gravity Forms and it works well. It creates the site and logs the customer in. I can have them click a link or button that I want to take them to the Pro Sites upgrade page they’d normally see after registering (at least when using the wp-signup.php?pro-site=1 URL). In other words, I want them to have to choose whether they want the free or upgraded plan.

I’ve tried sending the user to the URL you’d normally get when you click the Pro Sites “Account” menu item, which looks like this:

Then that redirects to a page like this:

That essentially works, but it’s not exactly the same page the user would normally see during the default registration process. They’d have to click to either upgrade or keep the basic, free plan. On the page above, it’s for changing the plan or payment details. The wording is different and there is no option to continue with the free plan.

The thing is, the URL looks the same either way. So the page at /pro-site/ shows something different if they’re clicking that link from the admin area. I was able to find in the code that it looks for that variable “psts-checkout” and if it’s set, it doesn’t show the normal upgrade message.

Is there a variable I could pass to the URL to tell it to show what it normally would right after registration?