Subscribe to- or remove users from MailPoet lists using the MailPoet API

functions.php

// Subscribe to or remove from MailPoet
// Add this at the bottom of your WordPress theme's functions.php file
// POST to https://headjack.io/wp-admin/admin-post.php?action=subscribe_to_mailpoet
function subscribe_to_mailpoet() {

	$api_key = "XXXXXXXXXX"; // Basic API key authentication, send over SSL

	if ( !empty($_POST['key']) && $_POST['key'] == $api_key ) {

		$subscriber_data = array(
		  'email' => sanitize_text_field($_POST['email']),
		  'first_name' => sanitize_text_field($_POST['first_name']),
		  'last_name' => sanitize_text_field($_POST['last_name'])
		);
		
		$options = array(
		  'send_confirmation_email' => false // default: true
		  //'schedule_welcome_email' => false
		);

		$default_list_id = array(11);

		// Check if subscriber exists
		try {
		  $subscriber = \MailPoet\API\API::MP('v1')->getSubscriber($subscriber_data['email']);
		} catch(Exception $exception) {
			// Subscriber does not yet exist, try to add subscriber
	  		try {
			  $subscriber = \MailPoet\API\API::MP('v1')->addSubscriber($subscriber_data, $default_list_id); // Add to default mailing list
			} catch(Exception $exception) {
			  echo json_encode($exception->getMessage());
			}
		}
		// Subscriber exists and needs to be added to certain lists
		if (!empty($_POST["add_to_lists"])) {
			$add_to_lists = array_map('intval', $_POST['add_to_lists']);
	  		try {
			  $subscriber = \MailPoet\API\API::MP('v1')->subscribeToLists($subscriber_data['email'], $add_to_lists, $options);
			} catch(Exception $exception) {
			  echo json_encode($exception->getMessage());
			}
	  	}
	  	// Subscriber exists and needs to be removed from certain lists
	  	if (!empty($_POST["remove_from_lists"])) {
	  		$remove_from_lists = array_map('intval', $_POST['remove_from_lists']);
	  		try {
			  $subscriber = \MailPoet\API\API::MP('v1')->unsubscribeFromLists($subscriber_data['email'], $remove_from_lists, $options);
			} catch(Exception $exception) {
			  echo json_encode($exception->getMessage());
			}
	  	}
	}
}
add_action('admin_post_subscribe_to_mailpoet', 'subscribe_to_mailpoet');
add_action('admin_post_nopriv_subscribe_to_mailpoet', 'subscribe_to_mailpoet');

Could not find API Key (on line 8) on Mailpoet.
Any possible advices? Thanks in advance!!