There may be a lot of players who buy the game activation code on Steam, but don’t know how to use it, look at this, maybe you will be enlightened.
How to Activate a Product on Steam?
The first is to open the Steam client App, find the upper left corner.
Then click on the “game” option and will jump out of menus.
Click on the “Activate the product on the steam“, after clicking it will jump out of a window.
Click Next, there will be another window, click to agree to activate the terms
Finally, enter the activation code in the input field (the CD Key code can be copied and pasted), you can redeem the game.