Mic level adjustment - how to do?

Hi. I've read throughout the forums and can't figure this out. I have the JBL OEM Radio/MFD/CD Player (GreenEdge) in my 2014 Prius 5. It's the one with 4 buttons on each side.

EJECT --... ...-- APPS
SEEK --.... ...-- SETUP
TRACK --... ...-- CAR
AUDIO --... ...-- PHONE (symbol)

I can't for the life of me figure out how to adjust the mic level/input gain. I've gone into maintenance/settings mode with the headlight flick trick, but the closest I can get to is "EXT BOX" which then has "Microphone Level Test" which just shows a 5-segment scale that shows the mic input level. I've gone into every option and can't find a place where there is the -5 to +5 mic input adjustment. Help! Thanks.

NB: If it's in the general Prius diagnostic mode, I have not tried that one yet.