If you purchased FS 13 on Steam, you generally do not need a manual key to install or play the game through the Steam client. However, if you are trying to convert your Steam version to a standalone version, or if a specific mod requires it, you can find your CD key:
An interesting technical feature of the Farming Simulator 2013 product key is that the activation status is stored as a hidden fs 13 product key
The key is typically printed on a sticker inside the DVD case, often located directly under the disc. If you purchased FS 13 on Steam, you
The safest place to find residual stock is on or Humble Bundle (though they rarely have legacy stock). Currently, the most reliable legitimate source is Amazon —specifically third-party sellers selling physical boxed copies. If you buy a physical box, the FS 13 product key is inside. Currently, the most reliable legitimate source is Amazon