12-11-2020 9:54 AM
eBay info says:
Go to the return request in My eBay and select the postage carrier from the dropdown list. If you select 'Other', enter the carrier name. Enter the tracking number. Select Mark as sent.
But if you used an eBay generated return postage label the tracking is added automatically.
If you have problems with that contact ebay and they may be able to add it for you. Use live chat at the very bottom of this: