parentWindow.html

<iframe id="verse-media-destination-frame" src="https://VERSE-MEDIA-PLAYER-IFRAME-URL"></iframe>

<script>
//Function to read value of a cookie
function readCookie(name) {
    var cookiename = name + "=";
    var ca = document.cookie.split(';');
    for(var i=0;i < ca.length;i++) {
        var c = ca[i];
        while (c.charAt(0)==' ') c = c.substring(1,c.length);
        if (c.indexOf(cookiename) == 0) return c.substring(cookiename.length,c.length);
    }
    return null;
}

//Call readCookie function to get value of user's Marketo cookie
var marketoTrackingId = readCookie('_mkto_trk');

// Gets a reference to the window object of the destination iframe.
var frameWindow = document.getElementById('verse-mediadestination-frame').contentWindow;

// Sends the client ID to the window inside the destination frame.
frameWindow.postMessage(marketoTrackingId, 'https://verse-media-video-player.com/video121312');

</script>

videoPlayeriFrame.js

window.addEventListener('message', function(event) {
  // Ignores messages from untrusted domains.
  if (event.origin != 'https://VERSE-MEDIA-PLAYER-IFRAME-URL') return;

  window.marketoTrackingId = event.data
});