Endwalker official early access starts on December 3rd, so if you want to purchase a preorder, I suggest doing it before early access is released. Full release is on December 7th, so if you want to wait to buy Endwalker (which comes with Shadowbringers) when it officially releases to the public who haven't preordered, you can then buy it on the 7th.

They currently are collecting payments for the preorder now though, considering the original early access date was November 19th. Hopefully this helps!