Will it be locked by platform? That is, if you preorder AOE4 on Steam, you’ll have the Steam version of Dawn of the Dukes and if you preorder it on MS Store, you’ll have the MS Store version?
You have to preorder AoE4 on the same platform you bought AoE2DE on.
Also I only know that the bonus is confirmed for steam. Feel free to check the MS Store page if the DLC is also included there ( i assume it is though)