Use another plugin's registration form to register user in PMPro

I’d like to use a different plugin’s registration forn because it’s got a nice Ajax-y user experience, but I also want my users to show up in my PMPro reporting dashboard as free users.

Is there a hidden field (or fields) that I can pass with this other plugin’s registration module that will register those users to my PMPro membership level 1?

Or is there a better method for doing this?

https://www.ads-software.com/plugins/paid-memberships-pro/