Auto enroll/add new user to specific member or product in Memberpress

auto_enroll.php

<?php

if(is_plugin_active('memberpress/memberpress.php')) {

  add_action( 'user_register', 'mp_auto_enroll' );
  //add_action( 'gform_user_registered', 'mp_auto_enroll', 10, 4 );
  
  function mp_auto_enroll($user_id, $user_config=array(), $entry='', $user_pass='') {
    $txn = new MeprTransaction();
    $txn->user_id = $user_id;
    $txn->product_id = 8; // TODO: Make sure you change this value to whatever the product_id will be
    $txn->trans_num  = uniqid();
    $txn->status     = MeprTransaction::$complete_str;
    $txn->gateway    = MeprTransaction::$free_gateway_str;
    $txn->expires_at = 0; // 0 = Lifetime, null = product default expiration
    $txn->store();
  }

}