popup.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Popup</title>
  </head>
  <body>
    <h1>Popup</h1>
    <form id="form">
      <input type="text" id="text" autofocus />
      <input type="submit" value="Send" />
    </form>
  </body>

  <script>
    const electron = require("electron");
    const { ipcRenderer } = electron;
    const form = document.getElementById("form");

    form.addEventListener("submit", (e) => {
      e.preventDefault;
      sendToMain();
    });

    const sendToMain = () => {
      const input = document.getElementById("text");
      const value = input.value;

      ipcRenderer.send("data-from-popup", value);
    };
  </script>
</html>