If you are buying this game you will be provided a product key for the same. Usually the package of the game includes the product key. You need to enter the product key at the time when you install the game on your computer. This will allow certain registry entries to be made. It also identifies the original user with the purchase code of the game. In certain cases the product key might not be available to the user. This happens when the game is downloaded online. Usually any legit purchase online will also come with a corresponding product key. If this is not supplied, one should seek this from the supplier or vendor. That will help one to enjoy all features of the game.
The product keys of the game usually come with the program. The series are also available for free download. This makes it easy for those who have purchased the game in a legitimate way, but have misplaced or did not receive the product key.
If you are trying to use the free version of the game, using any product key that you find online might not work. It would be best to opt for a legitimate version of the game and look for the corresponding product key.