If the label came from eBay it should upload tracking automatically, however if seller emailed a label to you then you may need to add it manually!
From eBay help pages:
Adding tracking to your return
Adding tracking details to your return helps protect against delays or issues in the refund process.
If you use an eBay return label, you don't need to do this – tracking details are uploaded automatically.
If you didn't use an eBay return label, here's how to add tracking information to your return:
Go to your Purchase history and find the item in Returns and cancelled orders.
Select View return details.
Select Mark as sent.
Select the carrier from the drop-down list, then enter the tracking number
If that doesn't work then you might need to try eBay chat and see if they can add the tracking number for you?
