Contact them this way.
https://www.ebay.co.uk/help/contact_us?id=4814&st=10
The contact links can be found on the left on that page.
Lines open today between 8 a.m. and 10 p.m.
However, it is not mandatory to use that code. Indeed, many sellers do not use the app to run their seller accounts, and that is needed here for that code.
Advise the seller to either go right of the item sold to More Actions, and there select Mark as Paid, and Mark as Dispatched.
Or, on the Seller Hub page, go to Orders>> Awaiting Postage >> Print Postage Label, click on the item, and you will see those instructions top left of the page.
You may need a desktop browser, not a mobile browser, or an app to manage this.
