It's not bindable by default. You need to go into DCS World\Mods\aircraft\AV8BNA\Input\AV8BNA\keyboard\default.lua (or "joystick" if you want to bind it to joystick)
And add the following:
{down = fcs_commands.Switch_ANTISKID, cockpit_device_id=devices.FLIGHTCONTROLS,value_down=0.0,name='Anti-Skid off',category='shortcuts'},
{down = fcs_commands.Switch_ANTISKID, cockpit_device_id=devices.FLIGHTCONTROLS,value_down=0.5,name='Anti-Skid on',category='shortcuts'},