Unifaun -

functions.php

<?php
/**
 * @snippet       WooCommerce - Unifaun - Set receiverReference tp order ID
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 4.1.1
 */
function redlight_unifaun_receiver_reference( $shipment, $order ) {
    $shipment['receiverReference'] = $order->get_order_number();
  return $shipment;
}
add_filter('ac_wc_unifaun_shipment_data', 'redlight_unifaun_receiver_reference', 10, 2 );