Add Order Number to Subject in Order administrator notification

-- HikaShop version -- : 2.6.4
-- Joomla version -- : 3.6.4

How can I add the ORDER NUMBER to the Subject in the Order administrator notification email?
I have been doing this by hacking the order.php file but have to re-do this each time there is an update.

The current SUBJECT shows: ORDER_ADMIN_NOTIFICATION_SUBJECT

The problem is that a lot of our clients use Google Apps in the Gmail interface which then combines all emails together that have the same SUBJECT line.

OK, so where is the LIST of Data Tags?

I did the change in the Overides:
New order #{order_number} has been created on your website.

This DID NOT add the Order Number to the subject line.
It simply produced: "New order # has been created on your website."

Prior to doing this I downloaded and installed the latest version of Hikashop.

I changed the Override to this and it worked fine: "New order %s has been created on your website."

The Subject of the Admin email is now: "New order B6D0D1 has been created on your website."

This is what I was trying to achieve.