Ordernumber in ExternalReference 1

functions.php

<?php
/**
 * @snippet       WooCommerce - Adds ordernumber to Fortnox ExternalInvoiceReference1 field
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.5.3
 */
function redlight_fortnox_order_external_reference( $orderData, $order_id) {
    $order = new WC_Order($order_id);
    $orderData['Order']['ExternalInvoiceReference1'] = $order->get_order_number();
    
    return $orderData;
}
add_filter('obj_fortnox_order_data', 'redlight_fortnox_order_external_reference', 10, 2 );