Ryanair booking for passenger with no surname on passport

Hi everyone,

I'm trying to book a Ryanair flight for my parents. Their passports only contain a given/first name and the surname/family name field is blank.

The Ryanair booking form requires both a first name and a surname. Has anyone dealt with this situation before? What should be entered in the surname field to ensure the booking matches the passport and avoids issues during check-in or boarding?

Any advice or personal experiences would be greatly appreciated. Thanks!