If you bought the game on Steam and need the actual serial key to register it on a third-party forum or mod site, you can usually find it by right-clicking the game in your Library > Manage > CD Keys . Why Version 1.138?
If you have the base retail version, you can still find official standalone patch files on reputable sites like ModDB to bring your game specifically to version 1.138. Experience the Great Deluge Safely
Authentic keys are usually tied to a specific Steam, GOG, or TaleWorlds account. Once used, they cannot be "re-verified" by another user.