Saw this on HotUKdeals, possibly a pricing error. You need Fifa 21 or 22 installed on your console, doesn't matter if it's your own bought copy or the Gamepass version. You will also need to have an active Gamepass subscription which is $1 to join and also lets you download Fifa 21 or 22 if you don't already have a copy.
The following doesn't work for PC or Playstation.
On the homescreen of Fifa 21/22 you will see an advert on the right advertising Fifa 23 at 40% off. Click on the offer and choose the World Cup version which says is an add-on for Fifa 23 and requires the base game to play. Continue onto the payment page and you'll see that you are getting the full standard version of Fifa 23 that includes the World Cup update for $23.99 down from $119.95
The discounted Xbox One version is £2 cheaper than the Series X/S version in the UK, but I don't have an Xbox One to find out the exact NZ price, it probably works out around $3/$4 cheaper but for now I'm just using $23.99 for all versions.
Giving this a go now, thanks OP. Will report back once I’ve gone through the process (Xbox One X owner here).
Also, I scrolled the HUKD thread for this deal and found a bit of info which applied to me:
“1. Delete FIFA 23 trial if you have it. Also delete the saved synced files on the xbox. You can find these in the "Manage game" section
2. Download FIFA 22 from the Game Pass library.
3. Before you start it, go to "manage game" and then delete the saved files on the xbox.
4. Start FIFA 22 and then try to access the offer again.”
Might save someone some potential wasted time if you’re trying to navigate trying this out too.