Visa waiver question?

My partner and i are planning a trip to NY in April, we have just gone to fill out the Visa Waiver form online via ESTA, it says we have to give details of where we are staying and flight numbers, we havent yet booked them as we wanted to be sure we would get in etc, plus we were waiting for a late deal, is this section compulsory ? Do we have to wait till we have booked out holiday before filling in the form?