<html>
      <head>
        <title>reCAPTCHA demo: Simple page</title>
        <script src="https://www.google.com/recaptcha/api.js" async defer></script>
      </head>
      <body>
        <form action="?" method="POST">
          <div class="g-recaptcha" data-sitekey="your_site_key"></div>
          <br/>
          <input type="submit" value="Submit">
        </form>
      </body>
    </html>