iFrame From Tracking With GTM by Post Message

iframe-form.js

// iframe code 
(function() {
  var form = document.querySelector('form[action="FormConfirmation.aspx?sourceformid=41&formid=42"]');
  if(form) {
    
    var queryString = form.getAttribute('action');
    form.addEventListener('submit', function (event) {

          var formData = new FormData(this);
          var userInputs = {};
      
          formData.forEach(function(value, key) {
              userInputs[key] = value;
          });

          window.top.postMessage(JSON.stringify(userInputs), 'https://www.main-website.com');
      });  
  }
})();


//website code
window.addEventListener('message', function (event) {
    if(event.origin !== 'https://iframe-site.com') return false;
    var data = JSON.parse(event.data);

    window.dataLayer = window.dataLayer || [];
    dataLayer.push({
        event: 'iframe_form_submit',
        formData: data
    });
});