If you are booking a JetStar flight you can request these while you are booking. This option shows after you have added your names to the bookings.
If you are booking on any other airline you can add a request in the notes or additional information box on the Travellers Details page at the time of making a booking
Please note many fare types do not allow pre-seating and this can be done at check-in only.
* If pre-seating is important to you please make sure you check the rules of the fare at the time of booking to ensure the fare type you are booking will allow this.