Add WooCommerce currency

functions.php

<?php
/**
* @snippet WooCommerce - Fortnox plugin by Redlight Media - Add WooCommerce currency via functions.php
* @author Redlight Media AB / Christopher Hedqvist
* @compatible WooCommerce 3.2.5
*/
function redlight_fortnox_currency($orderData, $order_id) {
    $orderData['Order']['Currency'] = get_woocommerce_currency();
    return $orderData;
}
add_filter('obj_fortnox_order_data', 'redlight_fortnox_currency', 10, 2 );