src/public/index.html

<html lang="utf-8">
  <head>
    <title>Webpush Server</title>
    <script>
      const sendEndpoint = "http://localhost:8000/send";
      const sendNotification = async () => {
        const data = document.getElementById("notification").value;
        const res = await fetch(sendEndpoint, {
          method: "POST",
          headers: {
            Accept: "application/json",
            "Content-Type": "application/json"
          },
          body: JSON.stringify(JSON.parse(data))
        });
        return res.status === 200 ? res.json() : false;
      };
    </script>
  </head>

  <body>
    <div id="root">
      <textarea rows="3" cols="70" id="notification">
{"title": "Notification title", "body": "View in my site.", "url":"https://mysite.com"}</textarea
      >
      <input
        type="button"
        onClick="javascript:sendNotification();"
        value="Send notification"
      />
    </div>
    <script src="/register.js"></script>
  </body>
</html>