Send data from form iframe using postMessage

gistfile1.txt

// somewhere inside iframe on succesfull form submit at https://websitewhereiframelives.com(Pardot)
  window.parent.postMessage({eventId: 'customEvent', data: {val1: '500'}}, "*");
  
// inside script on webpage
      window.addEventListener('message', function(message) {
        if(message.origin === 'https://websitewhereiframelives.com'){
          if(message.data.eventId === 'customEvent'){
            console.log(message.data.data.val1);
          }
        }
      })