Still don't follow
So you enter visa exempt 45 days.
Subsequently obtain a 30 day extension.
The flight out will not be required to obtain the extension.
The extension has nothing to do with flight out of Thailand.
An onward flight may be required by airline at departure when you fly to Thailand.
So again , why do you want the 30 day extension done straight away.
Some want to do that if staying somewhere remote.
Eg Obtain in Bangkok then travel to an island