Paid Custom Registration form, lets say product id XX

functions.php

add_filter('wplms_registration_form_submit_button',function($return){
  if(is_user_logged_in()){
      $check = wc_customer_bought_product('',get_current_user_id(),XX);
      if($check){
        return $return;
      }
  }
      
    return '<a href="'.get_permalink(XX).'">Purchase Membership $99</a>';
  
});