Go to Shipping > Order Summary.

 Use the available filters in order to find the invoice that you want to add the reference number too.

Under the Ref # column, click on the link Add Ref #.   

 In the pop-up window type in the "Air Way Bill" or "Reference #" and save it.

Once you are done, you will be able to see the "Air Way Bill" or "Reference #" on the printed invoice. In addition, if you go to  Shipping > Order Summary, you will see the reference number in the corresponding column.

On the Shipping > Order Summary screen use the filters to find the orders that you want to update.

 Select the orders by checking the check boxes, then go to the Actions button and click on the Set Ref # option.

 Enter the code and click on the Set Ref # green button.

 Once you are done, the system will update the selected orders with the information entered on the confirmation window.