Upvote:4
Few options for you in situation :
1/ Call the airline directly. They might be able to make a phone booking that will allow bypassing this online booking issue.
2/ Use a 3rd party website to perform the booking that also won't have this kind of problem.
I would really discourage you to input wrong details for the sole purpose of going through the booking process. You have a good probability to be caught and this might lead into problems. They will eventually deny boarding and even worse not refund your tickets. So to be on the safe side, call them or use an alternate website.