Campaign Monitor API add list subscriber via custom form in wordpress theme

cm-signup.php

<?php

add_action('tb_ajax_newsletter_signup', 'ajax_newsletter_signup');
add_action('tb_nopriv_ajax_newsletter_signup', 'ajax_newsletter_signup');

function ajax_newsletter_signup() {
	require_once '../libs/campaignmonitor/csrest_subscribers.php';

	$message_success = 'Thanks for signing up';
	$message_error_general = 'There was a problem with the newsletter signup submission. Please try again.';
	$message_error_email = 'Please enter a valid email address';
	$message_error_name = 'Please enter a valid name';

	if ($_SERVER['REQUEST_METHOD'] !== 'POST') {
		ajax_return_response(true, $message_error_general);
	} else {
		$api_key = get_option('options_campaign_monitor_api_key');
		$list_id = get_option('options_campaign_monitor_list_id');

		$name = array_key_exists('name', $_POST) ? $_POST['name'] : '';
		$name = clean_input($name);
		$email = array_key_exists('email', $_POST) ? $_POST['email'] : '';
		$email = clean_input($email);

		if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
			ajax_return_response(true, $message_error_email);
		} else if (empty($name)) {
			ajax_return_response(true, $message_error_name);
		} else {
			try {
				$auth = array(
					'api_key' => $api_key
				);
				$wrap = new CS_REST_Subscribers($list_id, $auth);

				$result = $wrap->add(array(
					'EmailAddress' => $email,
					'Name' => $name,
					'ConsentToTrack' => 'yes',
					'Resubscribe' => true
				));

				if ($result->was_successful()) {
					ajax_return_response(false, $message_success);
				} else {
					ajax_return_response(true, $message_error_general);
				}
			} catch (Exception $e) {
				ajax_return_response(true, $message_error_general);
			}
		}
	}
}

function ajax_return_response($error, $message) {
	header('Content-Type: application/json');
	$result = [
		'error' => $error,
		'message' => $message
	];
	echo json_encode($result);
	die();
}