get_mepr_id.php

<?php // Don't copy this line if adding this gist to a functions.php file.
      
function get_mepr_id() {
  if  ( !class_exists('MeprUtils')  ) {
    return 0;
  }

  $mepr_user = MeprUtils::get_currentuserinfo();

  if ( $mepr_user === false ) {
    return 0;
  }

  $active_subscriptions = $mepr_user->active_product_subscriptions('ids');

  if  ( empty($active_subscriptions) ) {
    return 0;
  }

  return $active_subscriptions[0]; // Returns the first found Active Membership ID

} // function()

add_filter('monsterinsights_frontend_tracking_options_persistent_gtag_before_pageview',
  function($options) {
    // Set the custom dimension
    $options['dimension9'] = get_mepr_id();
    return $options;
  },
  20 // priority
); // add_filter()