Jackbox game me a code, but can't find where to use it!

[pagerock]

Apologies from a noob: Jackbox discontinued a game I used to love and had paid for, so to make up for it they sent me a steam code to buy a newer game they have (Jackbox party pack). I just can't find where to enter it! It doesn't seem to be a steam wallet code, but nowhere else in the purchase process do I see any field to enter a discount code or something. Help please!

[General Zod]

In the Steam app, click on Games at the top, then Activate a game, then enter the code

[Cathulhu]

In the Steam Client, look in the bottom left corner, where it says "Add a game". Click it!