As an admin user you have the option to add your company codes to the system so that they show on the drop down menu and can be applied.

To do that you need to hover over your name in the upper right corner and go to Settings.
In Settings you will see Company Codes in the menu on the left.
In this screen you can see and managed all your company codes.
Press the dedicated button to add a new one.
In the pop-up you will need to select the type of code, type it in and hit Create.