Yes ...you can....Everything is Configurable!
reference_id is generated from line 161
checkout_payment.php and is inserted in TABLE 'orders' for each order.
The eTicket will reference this ID as a PlaceHolder %%Ref ID%%
$referenceID= substr(strtolower($order->customer['username']),0,5) . $order_timestamp ;
if (!$FSESSION->is_registered('referenceID')) $FSESSION->set('referenceID',$referenceID);
The default reference_id as of SEPT 2018
will create an ID beginning 5 letters of the customer 'username' and further expanded with an order timestamp.