You can use a Web Link for custom HTML pages.  Here’s a simple example you can try out with a button, text input, and message box.

Exercise:

Create a new Web Link, click the Switch View button in the menu bar ,  copy the code below,  click Save , then click Switch View  again to view the generated page:

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
  <title>Message Alert</title>
  <style>
    body {font-family: Arial, sans-serif; padding: 20px;}
  </style>
</head>
<body>
  <h1>Type a Message</h1>
  <input type="text" id="messageInput" placeholder="Enter your message" />
  <button onclick="displayMessage()">Show Message</button>
  <script>
    function displayMessage() {const input = document.getElementById("messageInput").value;
      alert(input || "Hello World");}
  </script>
</body>
</html>