Recording mic output as a separate track in Audacity

Ok, need help. I’m waiting for my new DJM900Nexus2. I do a live stream show and have to use the microphone a few times during the show. The show gets saved as audio and video but I need to be able to have the microphone routed to its own channel as I record the show in Audacity.

To the set up would be something like this

DJM mixer routing to
channel 1 mixer / track 1 + 2 in Audacity
channel 4 mixer / tracks 3 + 4 in Audacity

TRICKY PART
I need to have the microphone on its own track so when I mix down the audio I can mute the microphone so it is not part of the recording.

There has to be a way of doing this. Any help

Mac 10.14