Community Manager Ythisens has confirmed that the final part of the pre-patch event, The Siege of Lordaeron, will no longer need the pre-purchase of Battle for Azeroth to access and complete. Ythisens has also clarified that the https://bfa.wowhead.com/item=158075/heart-of-azeroth will be available as Battle for Azeroth launches.
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!
The Heart of Azeroth will be obtainable on the 13th at 3pm PDT. Essentially once the expansion goes live.
As we go into the final week of pre-patch events, it is worth noting that the http://www.wowhead.com/item=163127/smoldering-reins-of-the-teldrassil-hippogryph and http://www.wowhead.com/item=163128/war-torn-reins-of-the-undercity-plaguebat will be made unobtainable on the release of Battle for Azeroth.