If you ordered from Square-Enix store, you should have a confirmation email that looks like this. (This is the email I got).



Click the link highlighted here and it will bring you to a page that looks like this:



Your preorder activation code is displayed here. You then take this code and enter it into the heavensward preorder website.

This will generate another code which can then be redeemed by logging into the mogstation. This is probably the button that your friend was referring to.

If you never received an email like the one above then there's probably some mistake that needs to be resolved with SE customer support.