Purchasing the game through recognized platforms is the only secure way to obtain a valid key. Each platform has a specific method for retrieval:

: Some users on later versions (like 1.143) have reported being unable to enter keys. A common fix is using GOG Galaxy to revert to an older update to input the key before updating again. Are you having trouble with a specific error message during the activation process? Manual Activation - TaleWorlds Forums

There are two primary ways to register your serial key for version 1.138: Mount Blade With Fire And Sword 1.138 Serial Key - Facebook

: Version 1.138 marked the beginning of the "third version" of the game to hit the market, eventually evolving into the widely recognized version 1.143. Historical Foundation : Unlike other Mount & Blade titles that use the fictional world of Calradia, With Fire & Sword is loosely based on the historical novel by Henryk Sienkiewicz

Mount&BladeWith Fire and Sword key bug??? - TaleWorlds Forums

To install and activate (version 1.138), you will generally follow a standard installation process followed by an activation step using your serial key. Since version 1.138 is an older build, you may encounter specific activation hurdles. 1. Installation Steps

Depending on where you bought the game, you can find your key here: