You cannot assign a keyswitch to muting because it is not an on/off parameter. It is a varying control of a range of values that would require a corresponding type of message like those available (CC - continuous control or Aftertouch that varies based on pressure applied).
Note that keyswitch corresponds to MIDI notes, not keys on a keyboard (though I believe you mean you are using a function that is sending MIDI notes from your computer keyboard as is available in some audio programs like Logic Pro etc).
CC numbers are the MIDI CC number you are assigning to the control. There are some typically "reserved" for certain functions but others can be assigned and/or correspond to knobs, sliders, etc on your MIDI controller. For an overview of MIDI CC please check out this Sweetwater article:
https://www.sweetwater.com/insync/conti ... ontroller/