You can do this type of booking on the future.travel website. Simply put the birthdate as the estimated due date. Then once the child is born you may change the birthdate. You may not change the name.
You should select the type of seat for the child as infant in lap and they must travel with an adult.
Easier would be for the adult to book the ticket for the flights they want, and then add the infant after birth. The infant will not need a seat so therefore can be added at the last minute To the adult passengers seat assignment.
