How to redeem Build and Kill Zombies codes
There is no agreement on where the code box is. Four publishers give four descriptions, and one of them names a screen nobody else mentions.
Shop → EXCLUSIVE SHOP → COMMUNITY CODES → Redeem
The only account that names a specific screen. It also says most other sites copied a template and got this wrong.
Described by Aprasi
Open the Shop menu on the left, scroll down to the Codes section
No named sub-screen.
Described by Dexerto
Enter the shop hut, then scroll to the bottom of the shop window
Describes a building, not a menu.
Described by PCGamesN
Look for a Codes button, a gift icon, a bird icon or the settings menu
Four alternatives, none specific to this game.
Described by Lawod
Which to try first
The first one. A description that names an exact screen is harder to invent than a vague one, and a publisher that gets a menu name right has usually opened the game. The other three are the kind of wording that survives being copied between sites without anyone checking.
Two publishers also say a tutorial has to be finished before the shop opens at all. Neither shows it, so treat that as a thing to try rather than a fact.