add_filter('wcfm_shipment_tracking_fields', function($fields, $order_id, $order_item_id) {
    if(!$fields['wcfm_tracking_url']['value']) {
        $tracking_code = wc_get_order_item_meta($order_item_id, 'wcfm_tracking_code', true);
        $fields['wcfm_tracking_url']['value'] = 'baltazon.com/track-your-order/?tracking_id=' . $tracking_code;
    }
    return $fields;
}, 10, 3);