<form action="/login" method="post">
  <label for="username">Usuario:</label>
  <input type="text" id="username" name="username" required>

  <label for="password">Contraseña:</label>
  <input type="password" id="password" name="password" required>

  <!-- Aquí se incrusta el CAPTCHA -->
  <div class="g-recaptcha" data-sitekey="TU_SITE_KEY"></div>

  <button type="submit">Ingresar</button>
</form>

<!-- Script externo de Google -->
<script src="https://www.google.com/recaptcha/api.js" async defer></script>