[otrs] Customizing Subject Line of the Ticket

My objective is to change the subject line of the ticket to the customer ID. Explanation: When a customer logs in and creates a ticket, the subject line of that ticket should be the customerID of the customer who is creating the ticket. Is there a way we could do this through the web interface or does it require editing the source code, if so which of the file(s) needs to be edited. Thanks in advance. -- Roopesh