DOMSubTreeModified Event

 /* Using JQuery. In MailChimp case, since they assigned jQuery to $mcj
  * var $mcj = jQuery.noConflict(true);
  * replace jQuery with $mcj for best practice 
  */
  
 jQuery('#mce-success-response').bind("DOMSubtreeModified", function() {
   /* Do whatever you wish upon successful email submission */
   
   /* Setup Google Analytics Conversion Tracking */
   ga('send', {
     hitType: 'event',
     eventCategory: 'MailChimp',
     eventAction: 'Email Submit',
     eventLabel: 'Fall Campaign'
   });

   /* Redirect to a link of your choice with setTimeout */
   setTimeout(function() {
     window.open('http://google.com', '_blank');
   }, 3000);
 });
 
 /* Pure JavaScript version */
   document.getElementById('mce-success-response').addEventListener("DOMSubtreeModified", function() {
   /* Do whatever you wish upon successful email submission */
   
   /* Setup Google Analytics Conversion Tracking */
   ga('send', {
     hitType: 'event',
     eventCategory: 'MailChimp',
     eventAction: 'Email Submit',
     eventLabel: 'Fall Campaign'
   });

   /* Redirect to a link of your choice with setTimeout */
   setTimeout(function() {
     window.open('http://google.com', '_blank');
   }, 3000);
 });