Certificate of Authenticity (COA) sticker with a 25-character product key is located on the device. The labels are located in slightly different locations, including inside the battery compartment or on the bottom of the computer depending on which system type you own, example picture:

COA sticker may contain the concealment panel which consists of a small clear label with removable silver scratch-off material that has been applied to conceal a portion of the 25-characters of the product key.  Please use a finger nail to gently scratch the silver surface off. Do not try peel off the small clear label.