As far as I can see, there's no real way to communicate this preference to airlines. I won't book a flight on a MAX, but the airline can change the airframe at the last second and it's highly situational how willing/able I am to just refuse to fly if they swap in a MAX.
I've communicated it albeit very indirectly. When looking at holiday packages for this summer I looked up the route to see if it's a 737 max before booking.
