Shadowbringer XP Earring not working on HoH XP?

[Iscah]

You should receive a code after purchase (For PC, you'll get an email, for Steam it will be under "CD Key" option for the game. IDK about PS4)

PS4 is easy, at least if you're buying the digital version - I'd purchased the pre-order through PS Store, then yesterday when I logged in it prompted me to register the expansion to my account.

For someone who purchased a physical edition, they probably have to go through the same "registering the code on Mog Station" process you described.