Menu command name for adjusting brightness
Apple hasn't made this easy (or if they have, I've missed it).

The only way that I know to do this is with Keyboard Maestro. 

You may need to disable the iTunes media keys on your Mac keyboard:

launchctl unload -w /System/Library/LaunchAgents/com.apple.rcd.plist


And then you can use Keyboard Maestro’s Device Key option (for some reason the "Hot Key" didn't work.) Press the key that you want to use.