Apologies for the necro but I couldn't find a recent post on this. (Inb4 someone links one in this thread that I missed).
It was previously communicated that you would need to pre-purchase Battle for Azeroth to play the final part of the 8.0 pre-patch event tomorrow, The Siege of Lordaeron, however, this is no longer the case and we have changed it to now be available regardless of prepurchasing the expansion!
I also want to throw out a reminder that you all have a week to go before the pre patch events are removed from the game and as such the mounts, Teldrassil Hippogryph and the Undercity Plaguebat for the Alliance and Horde respectively, will be no longer attainable. So remember to get out to Darkshore and get those done so you don't miss out!