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jQuery(document).ready( function($){
    //Snag the URL of the iframe so we can use it later
    var url = $('.optin_custom_html_applied iframe').attr('src');

    $(document).on('OptinMonsterOnClose', function( event, data, object){
      //Assign the iframe's src to null, which kills the video
      $('.optin_custom_html_applied iframe').attr('src', '');
    });
});