How to get a postMessage message from Duda into the embedded iframe?

From what I have read, having a client-side code read the message from the iframe outside of the sandboxed one seems impossible. I can only have it sent if to the iframe outside the sandboxed. What would be the solutions, then?
Here is what I have tried so far:

 document.addEventListener("DOMContentLoaded", function() {
   const queryString = window.location.search;
   const urlParams = new URLSearchParams(queryString);
   const signupToken = urlParams.get('token');
   //const iframe = document.getElementById('myiframe'); //Most outter iframe not reachable
   //const iframe = document.getElementById('sandboxed'); //Outter iframe not reachable
   const iframe = document.getElementById('userHtmlFrame'); //This would be the ideal, but not reachable
   if (iframe && signupToken) {
     console.log("WebApp iframe found and here is the token " + signupToken)
     iframe.contentWindow.postMessage({
       signupToken: signupToken
     }, "*");
   }
 });

Then, to read the message, I have:

let urlToken = '';
document.addEventListener("DOMContentLoaded", () => {
  window.addEventListener('message', (event) => {
    console.log("Received message:", event);
    if (event.origin === "https://www.kshkjfhkajhfd.com") {
      console.log("Correct origin");
      if (event.data && event.data.signupToken) {
        urlToken = event.data.signupToken;
        console.log("Token received from parent:", urlToken);
      } else {
        console.log("Message does not contain expected data:", event.data);
      }
    } else {
      console.error("Message from unknown or incorrect origin:", event.origin);
    }
  }, false);
});