I've found the answer and solved the problem.
I edited the file /usr/share/X11/xorg.conf.d/40-libinput.conf and I changed the following section (the one having pointer identifier), adding the two Option lines:

Section "InputClass"
        Identifier "libinput pointer catchall"
        MatchIsPointer "on"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "ScrollMethod" "button"
        Option "ScrollButton" "2"
EndSection