Export custom meta from Woocommerce to Shipstation - php

Question

I have a problem with exporting products to shipstation, the fields that are generated using the Product Add-on Ultimate plugin are not exported to shipstation
How can I put together the correct function for exporting additional fields??
// Add this code to your theme functions.php file or a custom plugin
add_filter( 'woocommerce_shipstation_export_custom_field_2', 'shipstation_custom_field_2' );
function shipstation_custom_field_2() {
return '\_meta_key'; // Replace this with the key of your custom field
}
// This is for custom field 3
add_filter( 'woocommerce_shipstation_export_custom_field_3', 'shipstation_custom_field_3' );
function shipstation_custom_field_3() {
return '\_meta_key_2'; // Replace this with the key of your custom field
}
I found this solution in plugin documentation but it doesn't work for me
<?php
/**
* Get add-on metadata from each line item in the order
* #param $order_id
* #param $metakey The add-ons metakey (field label), usually prefixed by an underscore
*/
function prefix_get_addons_metadata_by_key( $order_id, $metakey=false ) {
$order = wc_get_order( $order_id );
$order_line_items = $order->get_items();
foreach( $order_line_items as $line_item ) {
// Here, we can iterate through all the meta for this line item
$all_meta = $line_item->get_meta_data();
if( $all_meta ) {
foreach( $all_meta as $meta ) {
$meta_id = $meta->id;
$meta_key = $meta->key;
$meta_value = $meta->value;
}
}
// Here, we can get the value by a specific metakey
if( $metakey ) {
$meta_value = $line_item->get_meta( $metakey );
}
}
}
This is from the plugin side.