https://wiki.debian.org/SynapticsTouchpad
Instructions for stretch (Debian 9) should also work for buster.
You are right. This did it for me:
$ mkdir -p /etc/X11/xorg.conf.d
$ echo 'Section "InputClass"
Identifier "libinput touchpad catchall"
MatchIsTouchpad "on"
MatchDevicePath "/dev/input/event*"
Driver "libinput"
Option "Tapping" "on"
EndSection' > /etc/X11/xorg.conf.d/40-libinput.conf
$ systemctl restart lightdm
Thank you very much.