Fortnox - External Invoice Reference

functions.php

<?php
/**
 * @snippet       WooCommerce - Adds ordernumber to Fortnox ExternalInvoiceReference2 field
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 4.5.0
 */
function redlight_fortnox_invoice_order_reference( $orderData, $order_id) {
    $order = wc_get_order($order_id);
    $orderData['Order']['ExternalInvoiceReference2'] = $order->get_order_number();
    
    return $orderData;
}
add_filter('obj_fortnox_order_data', 'redlight_fortnox_invoice_order_reference', 10, 2 );