How to control the Master Stereo out using external device and only him ?


i own several control device, (almost all the CMC series (PD, 2xFD, TP, CH, QC) but as well a iRig Keys 37 PRo with a button sending volume 7 CC (but can send anything else as CC), and a shuttleeXpress.

I try to control the stereo out (the master out) in the mixboard of cubase 9.5 and only Him not any other fader ! only the stereo out !
but can’t find a specific CC or anything else in the shortcuts to control only the stereo out all the time.

anybody know how to do it ? Thank you !