Just to clear up any confusion, follow these steps:

1) Fill in Passport information normally
2) For VISA part, use “0” as the number and a date a month from now for the date field.
3) For the alternate document section use your KTN

Works like a charm... they probably contracted their website design from a third party years ago and have no in-house staff capable of updating it.
As with everything else IBERIA, it’s a sucky experience from start to finish ;-)