You need to manually now mark the item as dispatched and in the same place you can add that tracking number.
You need to finalise the transaction now only on the Seller Hub here:
https://www.ebay.co.uk/sh/ovw
Go to Orders >> Awaiting Postage, click on the item then top left on View Order Details.
Far right from More Action drop down menu click on ' Mark as Dispatched' if you have it, add any tracking number there too.
Please be aware, you do not need to buy eBay postage, it's always fine to buy over the counter postage at your Post Office, then just mark as dispatched as above.
@pat5584
