How do I handle flight booking for a 60-day visa when I intend to stay 90 days in Thailand?

For a 60-day setv, where is duration of staying do I say 90 days ?

Because my intention is to extend the 60 days by another 30 days.

Do I book and put the flight information on which is a 90 day return ticket ? How do I have to book a 60-day return flight then change my flight to the 90 days incuring more costs?

Thank-you