WIZ does not accept 9999 as the co-borrower credit information even when there is no co-borrower for withdrawn applications. Is reporting 8888 (Not Applicable) for both borrower and co-borrower (even if there is not one), the correct reporting method?

Borrower - 8888
Co-Borrower - 8888

Is this the appropriate way to report this?

Yes, that's correct. This is one of the weird HMDA requirements. Even if there is no co-borrower, you always report 8888 for the credit score and 9-NA for the credit score model on withdrawn and closed for incompleteness.