Not sure if you’d already resolved the issue, but in case you haven’t...

There’s a pink ”Enter Expansion Registration Code” button under Your Account tab, which is not shown in the guide. (The button is above subscription renewal section).

On the page it leads to it showed
A license for the following products can be registered on this page:

FINAL FANTASY XIV: Shadowbringers Standard Edition Windows® / Mac
FINAL FANTASY XIV: Shadowbringers Collector's Edition Windows® / Mac
FINAL FANTASY XIV Online Complete Edition Windows® / Mac
FINAL FANTASY XIV Online Complete Collector's Edition Windows® / Mac
Is that the page where you tried your code on?