There is a reservation desk to the Specialty restaurants and a # you can call.
Charge is added to your onboard account. The wait staff is included in the $12 a day service charge. That said it very common for most to leave a few dollars extra for exceptional service but not expected. I usually leave an extra $10, some more, some less, some none. Your call. Cash is king but you can add a little to the bill and it goes on your account as well.