There are 2 ways to integrate the Restrict Content Pro.

1st: streamless – replace the plugin registration form with a Restrict Content Pro.

2nd: use the default form but after redirect to the Restrict Content Pro registration page where the user could select his plan:

Advanced tweaks

1. To open the log-in tab while clicking on the “Log in to renew or change an existing membership.”  add this:

   

2. To hide the Register New Account heading use this CSS:
   .lrm-form h3.rcp_header { display: none; }
   https://wplift.com/how-to-add-custom-css-to-wordpress