To create a GL Code for use within Staylist Pro for accounting purposes:
In the main settings cogwheel, navigate to Accounts -> Ledger Accounts.
Click on the New/+ button in the upper-right corner.
Enter in a name for the GL Code.
Enter in the desired GL Code in the Code box.
Enter in a GL Code description in the Description box if desired.
Be sure to save!
After these steps are complete, you should be able to assign this GL code wherever necessary.