Most downloadable "key generators" are disguised Trojan horses. Running these executables can infect your computer with spyware, adware, or ransomware that steals your personal data.
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: fs 13 product key