You can add a code in either of two ways:
Open the project and go to the Project Info screen.
Hover over the Project Info icon and select the option to add a code.

Alternatively:
Open the project.
Select the three-dot menu in the project navigation.
Choose Add project code.
Enter the code and save your changes.
Once added, the code will be visible in:
Project cards
The Project Overview screen
The Project Info screen and overlay
My Tasks