When you try to define Exchange Server 2016 servers as a new download product update in WSUS, you could choose Exchange 2016 Server as a product or classification in the WSUS console.
In the WSUS console Microsoft's latest messaging server version, Exchange Server 2016, is not be offered as an product or classification to be defined for updates. After the last patchday on 10/13/2015 the server will also not shown as a selectable product in WSUS.
Microsoft Exchange Sevrer 2016 RTM version was released on 1st of October 2015, and no official, dedicated Exchange 2016 server update was delivered in the meantime. That could indicate why the server is not listed in the moment. Furthermore it is also conceivable that the server will receive its first update on the activated E2013 update requirements, and afterwards it will be maybe shown as a own product in the WSUS product list.