You should start looking in Mods\aircraft\FA-18C\Input\FA-18C\joystick\default.lua
Looking for ATC... bingo!
{ down = hotas_commands.THROTTLE_ATC, up = hotas_commands.THROTTLE_ATC, cockpit_device_id = devices.HOTAS, value_down = 1.0, value_up = 0.0, name = _('ATC Engage/Disengage Switch'), category = {_('Throttle Grip'), _('HOTAS')}},
So, ATC is one of devices.HOTAS commands.
For Device ID, look into \Mods\aircraft\FA-18C\Cockpit\Scripts\device.lua
devices["HOTAS"] = counter()--13 -- Stick and throttle grips,
The DeviceID is assigned by counter() auto-inc function, but the comment at the right shows
that it's 13.
Then, open \Mods\aircraft\FA-18C\Cockpit\Scripts\command_defs.lua,
and look for THROTTLE_ATC.
We once again have this counter() function... But in the beginning of each commands set the counter
is reset to 3000 (count = start_command).
Counting down to THROTTLE_ATC... The CommandID seems to be 3037.
So... DeviceID = 13, CommandID = 3037