I couldn’t figure out how to do it with input-remapper. But while I was searching for that, someone pointed this out:
I ended up finding a tip that said I could set this in gsettings:
gsettings set org.gnome.desktop.peripherals.trackball scroll-wheel-emulation-button 8
I can also use dconf:
dconf write /org/gnome/desktop/peripherals/trackball/scroll-wheel-emulation-button 8
Bluefin also has a dconf editor GUI that works for this same thing.
