Stop a video from playing when the optin closes, then add the video url back when the optin opens - useful for Manual Trigger optins.

OptinMonsterOnShow.js

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('OptinMonsterOnShow', function( event, data, object){
      //Below we remove the URL to stop the video from playing, here we add it back in
      $('.optin_custom_html_applied iframe').attr('src', url);
    });

    $(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', '');
    });
});