On the square enix account website.
Go to Services and Options
Pick Select Service
At the top are two big buttons.
One tells you to add codes to existing POL service accounts.
One gives you the option of entering the code to create a new POL service account.
If you add codes to an existing account, you enter in the code, and then select the account you want to apply it to.
If you need to create a new account from a Steam code, you pick "Create a new POL service account."
SE did sort of hide them, and they look less like buttons and more like random pictures at the top of the page.