The WAYBILL-number is a series of numbers that can be found on the shipping label, that is generated in example for your return. You will receive this document via e-mail after your return request has been approved.
The photo below shows exactly where on the shipping label the WAYBILL can be found.
It is located below the first bar code of the shipping label document. This number is also the tracking for the parcel, so if you want to check your return's current status, you can just insert the WAYBILL to the DHL tracking locator here and see the all the updates regarding it. You can leave out the WAYBILL-prefix and just insert the numbers.
To locate back to the return instructions, click here.