Add the required link attributes to a WordPress menu item using Javascript. Useful if the site uses a custom walker class.

om-monsterlink-wp-menu.php

add_action( 'wp_footer', 'om_dynamic_monsterlink_wp_menu' );
/**
 * Dynamically adds the required attributes to the menu item #4632 link to trigger the specified MonsterLink
 * IMPORTANT - replace '4632' below with the ID of your specific menu item
 */
function om_dynamic_monsterlink_wp_menu() {
  echo '<script type="text/javascript">jQuery(document).ready(function($){
    $(".menu-item-4632 a").attr("class", "manual-optin-trigger");
    $(".menu-item-4632 a").attr("data-optin-slug", "YOUR_OPTIN_SLUG_HERE");
  });</script>';
}