Melbourne to Sydney

Next month I will be flying on UA840 from Sydney to LAX. I just found out I will need to be in Melbourne before that flight so I'll need to travel from Melbourne to Sydney. I know the UA840 flight originates in Melbourne but when I searched for the flight it wasn't found for booking.

Anyone know if I can book this flight? Do I need to call? It would be nice to be on the same plane and not have to worry about changing terminals in Sydney.

Thanks.