Oh I forgot one thing, , you need to add ",d.*" like this:
$database->setQuery('SELECT a.*,b.*,c.*,d.*'.$query,(int)$pageInfo->limit->start,(int)$pageInfo->limit->value);
in the file: "yourSite/administrator/components/com_hikashop/views/order/view.html.php and in the function listing().
Add "d.order_product_code" in the searchmap:
$searchMap = array('c.id','c.username','c.name','a.user_email','b.order_user_id','b.order_number','b.order_id','b.order_full_price','d.order_product_code');
And then you can filter by product code in the filter field on the left.