It will be included with your WoW subscription.
The BlizzCon 2018 Opening Ceremony saw the announcement that World of Warcraft Classic is arriving next Summer and will be available to anyone with a World of Warcraft subscription.
— World of Warcraft (@Warcraft) November 2, 2018
World of Warcraft Classic was announced at BlizzCon 2017 – much to everyone’s surprise – and this year sees a playable demo, available both on the show floor at BlizzCon, and at home for Virtual ticket holders.
We’ll be sure to update you the second we have a firm release date.