Integrate Google Recaptcha v2 into custom AJAX form (Wordpress)

functions.php

<?php

// First we register the Google Recaptcha script as a dependency with an onload callback
wp_register_script( 'recaptcha_v2', add_query_arg(['render' => 'explicit', 'onload' => 'onloadRecaptchaCallback'], 'https://www.google.com/recaptcha/api.js'), [], null, true);
wp_enqueue_script( 'recaptcha_v2' );


// Here we validate Google Recaptcha token issued on our frontend with Google servers using our custom 'validate_captcha' WP AJAX hook
function hh_validate_captcha() {
  check_ajax_referer( 'hhsubscribe', '_ajax_nonce' );
  if ( 'POST' == $_SERVER['REQUEST_METHOD'] && isset( $_POST['fe_token'] ) && !empty( $_POST['fe_token'] ) ) {
    $fe_token  = $_POST['fe_token'];
    $response = file_get_contents( 'https://www.google.com/recaptcha/api/siteverify?secret=' . get_option( 'rg_gforms_captcha_private_key' ) . '&response=' . $fe_token . '&remoteip=' . $_SERVER['REMOTE_ADDR'] );
    $obj = json_decode( $response );

    // Return the verification response for our JS to handle
    wp_send_json( $obj, 200 );
  } else {
    wp_send_json( ['message' => 'Submission method not POST or captcha blank'], 403 );
  }
}

add_action( 'wp_ajax_validate_captcha', 'hh_validate_captcha' );
add_action( 'wp_ajax_nopriv_validate_captcha', 'hh_validate_captcha' );



// Here we do usual AJAX-enabled form sending stuff - saving form fields to a WP post, posting to an API, etc.
function hh_newsletter_subscribe() {
  if ( !isset( $_POST['hhsubscribe_nonce'] ) || !wp_verify_nonce( $_POST['hhsubscribe_nonce'], 'hhsubscribe' ) ) {
    wp_nonce_ays( '' );
  }

//   $response = hh_newsletter_sendapplication( $_POST['email'], $_POST['firstname'], $_POST['lastname'] );
//   if ( $_POST['email'] && 200 == $response['response']['code'] ) {
//     echo 'Success!';
//   } else {
//     echo 'Error';
//   }

  die;
}

add_action( 'wp_ajax_hhsubscribe', 'hh_newsletter_subscribe' );
add_action( 'wp_ajax_nopriv_hhsubscribe', 'hh_newsletter_subscribe' );

index.php

<script>
  // A script that sets a DOM element to attach the captcha to and a callback to execute after a frontend captcha token has been created 
  function onloadRecaptchaCallback() {
    grecaptcha.render('hhsubscribe_submit', {
      'sitekey' : <?php echo get_option( 'rg_gforms_captcha_public_key' ); ?>,
      'callback' : onRecaptchaFETokenCreated
    });
  }
  // Here we make a request to out WP AJAX action that verifies the FE token with Google servers using our private key and outputs either pass or fail 
  function onRecaptchaFETokenCreated(recaptchaFEToken) {
    const $form = document.getElementById('newsletter-form');
    const nonce = $form.querySelector('[name=hhsubscribe_nonce]').value;

    fetch(script_data.ajaxurl, {
      method: 'post',
      body: '_ajax_nonce=' + nonce + '&action=validate_captcha&fe_token=' + recaptchaFEToken,
      credentials: 'same-origin',
      headers: {
        'Content-Type': 'application/x-www-form-urlencoded; charset=utf-8'
      },
    })
    .then(response => response.json())
    .then(data => {
      if (data.success) {
        const request = new XMLHttpRequest();
        request.open('POST', $form.getAttribute('action'), true);
        request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
        const form_data = new URLSearchParams(Array.from(new FormData($form))).toString();
        request.send(form_data);

        request.onreadystatechange = function() {
          if (request.readyState === 4) {
            document.getElementById('newsletter-form-output').innerHTML = request.responseText;
          }
        };
      } else {
        if (data.message) {
          console.log('Validation error! ', data.message);
        }
      }
    })
    .catch(function() {
      document.getElementById('newsletter-form-output').innerHTML = 'reCAPTCHA verification failed, please try again';
    });
  };
</script>
<form action="<?php echo admin_url( 'admin-ajax.php' ); ?>" method="post" id="newsletter-form">
  <input type="hidden" name="action" value="hhsubscribe">
  <?php wp_nonce_field( 'hhsubscribe', 'hhsubscribe_nonce', false ); ?>
  ...
  <button type="submit" id="hhsubscribe_submit">Submit</button>
  <output id="newsletter-form-output"></output>
</form>