Redirect when a specific optin is closed using the OptinMonster event 'OptinMonsterOnClose'.

om-custom-redirect-close.js

jQuery(document).ready(function($){
    function omRedirect() {
          var url = "http://optinmonster.com";
          $(location).attr('href',url);
     }
     $(document).on('OptinMonsterOnClose', function(event, data, object){ 
       if(data.optin = 'YOUR_OPTIN_SLUG_HERE') {
         omRedirect(); 
       }
     });

});