Hello everybody, i’m new on this forum.
With what: as Paul said, you need an usb device.
How to: click interface / click midi control and select your device / click midi learn / click the appropriate assign button / manipulate the controller you want to operate / close the interface window: it’s done!
I use an Audiofront Quattro box to plug my footswitches and expression pedals.
It is an affordable and efficient device with a software to program the pedals you connect.
It converts analog to usb, allowing to trig in MG with specified functions and with settings storable.
https://www.audiofront.net/MIDIExpression.php
I use it in live performances, mixing guitar/synth or else with 2 expression pedals, changing patches with a 2 buttons footswitch, another for various purposes ie: changing the leslie rotary speed, etc.
In this way, MG ist a really cool machine on stage.