I would go to Help > Configuration Assistant, choose "Other" from the bottom of the list.
Then enter RepRap Marlin firmware
Build dimensions: 210 x 210 x 210
1 Extruder
Nozzle diameter: 0.4mm
Filament diameter: 1.75mm
Then click next and the software will automatically create a profile for you based on those settings. The A9 is a pretty simple machine, so that should give a good starting point.