WHMCS doesn't currently support additional domain fields for transfers. This means that transfers for .es domains (and probably many more) can't be automated, because the customer is never asked to fill the necessary fields. WHMCS should implement required domain fields in both domain registrations and transfers.
Transfer domains additional fields like domain register availability
We need availability for define additional fields like domain holder in domain transfer page.WHMCS supports domain register additional fields like terms and conditions but this is not defined for transfer page.