This is what the Subaru "All in One" key looks like.
If you already have two keys programmed to the car, programming instructions for an additional key are outlined in the manual. You'll need both keys in hand to program the 3rd key.
If you only have one key, the dealer will need your security code (located on a very small metal tag which should have came with the keys when you purchased the car), and would have to use the Subaru Select Monitor scan tool to program the new key to the car. If you don't have the security code, the dealer can get that info from SoA, for a small cost.