[itsm] How to assine SLA to Ticket from Agent Login

Hi Team, I am working on OTRS:ITSM 2.4. I have a query in regards with SLA. I can configure SLA from 'Admin' for a perticular service. Then I believe that when customer selects a service while creating a new ticket, the SLA should get selected for that perticular service. But instead the customer needs to select the SLA for his selected service. Can anyone throw light on this. Regards, Amol.