Please note: Giants keys are not Steam keys.They will not activate on Steam.
In order to redeem your GIANTS key, you will need to visit the GIANTS Software website here:
https://eshop.giants-software.com/downloads.php , enter/copy your key and click on Download. You will then have access to setup files. Download and install.
If your key is for Farming Simulator DLC, please visit the following site here: https://www.farming-simulator.com/extraContent.php