reCAPTCHA

 <script src="https://www.google.com/recaptcha/api.js"></script>
 <script>
   function onSubmit(token) {
     document.getElementById("demo-form").submit();
   }
 </script>
<button class="g-recaptcha" 
        data-sitekey="reCAPTCHA_site_key" 
        data-callback='onSubmit' 
        data-action='submit'>Submit</button>