<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Popup Window</title>
  <style>
    #formbox {
      padding: 20px;
      border: 1px solid #999999;
    }
  </style>
  <script>
    function initiate() {
      const button = document.getElementById("button");
      button.addEventListener("click", send);
    }
    function send() {
      const message = document.getElementById("message").value;
      window.opener.postMessage(message, "https://www.domain1.com");
    }
    window.addEventListener("DOMContentLoaded", initiate);
  </script>
</head>
<body>
  <section id="formbox">
    <form name="form">
      <label for="message">My Message: </label>
      <input type="text" name="message" id="message" required>
      <button type="button" id="button">Send</button>
    </form>
  </section>
</body>
</html>