What should you do when you are buying a ticket for someone with only one name (i.e. no family name, last name, or surname)? Some airlines tell you to use an honorific like "MR" or "MS" in place of the first name, but visas are generally issued to First Name Unknown, "FNU".
The 2023-2024 Airline Industry Reservations Interline Message Procedures (archive link) issued by the IATA specify that you should write your passenger's name as the last name and use "FNU" (standing for “First Name Unknown”) in place of the first name. It also states that honorifics are no longer part of the official standard. (This is good, because honorifics have caused problems.)
As mentioned in this past question, your travel is at the mercy of both the airline and the immigration service of the destination country, so it may help to check with both. While my airline told me via email that they prefer an honorific, they acknowledged that the IATA ruling supersedes it. Other airlines might demand an honorific, as silly as that sounds...