I suggest adding Crysta, but doing it through UltimatePay. You can do that by going to the account management site (account.square-enix.com), logging in, clicking the "Add Square Enix Crysta" option in the top left, and then choosing your Crysta amount. The payment option UltimatePay shows up below the one that says "Credit/Debit Card"

If it doesn't show up, you may want to make sure you're using Internet Explorer or FireFox