Guidance on how promo/voucher codes work on PeoplePerHour:
The x% vouchers only give a discount on the deposit value NOT the invoice values. So if you would like to use a code that has been provided, please make sure that it is applied when you are paying for a deposit. When an invoice is raised, there will not be an option to enter the code.
The photo below shows where the code enter option is located. When you select to pay for a deposit, the option is available in the upper right-hand corner as shown in the red circle in the screenshot.
Promo codes cannot be added manually if they are not added in the initial deposit phase. They can, however, be used on the next job that you would like to use it for.