forms/html-invisible-recaptcha.html

<head>
    <script src='https://www.google.com/recaptcha/api.js'></script>
    <script>
        function onSubmit(token) {
          document.getElementById("contact-form").submit();
        }
      </script>
</head>
  <body>

    <form class="contact-form" id="contact-form" name="contact-v3" method="POST" netlify-recaptcha netlify-honeypot="required_name" netlify>
    <div class="field-body">
        <p>
                <label>Don't fill this out if you're human: <input name="required_name" /></label>
        </p>
    </div>
    <div class="field is-required">
        <label class="label" for="contact-email">
            Email
        </label>
        <div class="control">
            <input class="input" type="email" id="contact-email" maxlength="80" name="email" required>
        </div>
    </div>
    <div class="field is-required">
        <label class="label" for="contact-body">
            Message
        </label>
        <div class="control">
            <textarea class="textarea" id="contact-body" name="message" required></textarea>
        </div>
    </div>

    <p>
        <small class="p--fine-print recaptcha-terms">
            Protected by reCAPTCHA. See
            <a class="p__a" href="https://www.google.com/intl/en/policies/privacy/" target="_blank" rel="external">Privacy Policy</a>
            and
            <a class="p__a" href="https://www.google.com/intl/en/policies/terms/" target="_blank" rel="external">Terms of Use</a>.
        </small>
    </p>

    <div class="control">
        <button
        class="g-recaptcha"
        data-sitekey="YOUR_SITE_KEY_HERE"
        data-callback="onSubmit">
        Submit
        </button>
    </div>
</form>


  </body>
</html>