<?php

add_action( 'mailster', function() {
  function mytag_function($option, $fallback, $campaignID = NULL, $subscriberID = NULL){
  	​$referral = new WPGens_RAF_User($subscriberID);
	$referralID = $referral->get_referral_id();
	​$link = get_home_url().'?raf='.$referralID;
	return $link;
  }
  mailster_add_tag('referral_link', 'mytag_function');
});