hubspot-form-event-listener.js

<script type="text/javascript">
  window.addEventListener("message", function(event) {
    if(event.data.type === 'hsFormCallback' && event.data.eventName === 'onFormSubmitted') {
      window.dataLayer.push({
        'event': 'hubspot-form-success',
        'hs-form-guid': event.data.id
      });
    }
  });
</script>