Add custom field to customer GUEST USERS

functions.php

<?php
 /**
 * @snippet       WooCommerce - Fortnox plugin by Redlight Media - (GUEST USERS) Set $our_custom_field as Comments on Customer
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.4.2
 */
function redlight_fortnox_guest_customer_data( $customer, $order_id ) {
        $order = wc_get_order( $order_id );

        $our_custom_field = $order->get_meta('our_custom_field');
        if(isset($our_custom_field)){
                $customer['Customer']['Comments'] = $our_custom_field;
        }

    return $customer;
}
add_filter( 'obj_fortnox_guest_customer_data', 'redlight_fortnox_guest_customer_data', 10, 2 );