Klarna External references Fortnox

functions.php

<?php
/**
 * @snippet       WooCommerce - Adds Klarna Invoice and ordernumber to Fortnox ExternalInvoiceReference1 & ExternalInvoiceReference2 fields
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.3.5
 */
function redlight_fortnox_klarna_invoice_order_reference( $orderData, $order_id) {
    $order = new WC_Order($order_id);
    $klarna_invoice_number = get_post_meta( $order->id, '_klarna_invoice_number', true );
    $klarna_order_reservation = get_post_meta( $order->id, '_klarna_order_reservation', true );
    $orderData['Order']['ExternalInvoiceReference1'] = $klarna_invoice_number;
    $orderData['Order']['ExternalInvoiceReference2'] = $klarna_order_reservation;
    
    return $orderData;
}
add_filter('obj_fortnox_order_data', 'redlight_fortnox_klarna_invoice_order_reference', 10, 2 );