GTM Custom HTML Tag: iFrame Listener for Simplybook

tagcode.js

<script>
  function bindEvent(element, eventName, eventHandler) {
    if (element.addEventListener){
      element.addEventListener(eventName, eventHandler, false);
    } else if (element.attachEvent) {
      element.attachEvent('on' + eventName, eventHandler);
    }
  }

  bindEvent(window, 'message', function (e) {
    if (!e.data) return;
    var ev = e.data.event;
    if (ev == 'stepChanged' || ev == 'bookingDone') {
      window.dataLayer.push({
        event: ev,
        bookingStep: e.data.step
      });
    } 
  });

</script>