om-custom-redirect-close.js

jQuery(document).ready(function($){

    function omRedirect() {

          var url = "https://optinmonster.com";

          $(location).attr('href',url);

     }

     $(document).on('OptinMonsterOnClose', function(event, data, object){ 

       if(data.optin = 'YOUR_OPTIN_SLUG_HERE') {

         omRedirect(); 

       }

     });

});