Who are you saying this to? Are you pretending to be John Cena? If not, who is he and why are you telling someone there is a room booked for him?
You can use either preposition there but it's still not a natural sentence without context.
He would probably say to the receptionist "I have a reservation. My name is John Cena". I don't know why you said that you didn't mean to write "Cena". He would need to give his surname to the receptionist. There's no point in him saying "I'm John. I have a reservation".
Did you read the fourth sentence in post #2?