You should create this file /etc/X11/xorg.conf.d/50-synaptics.conf with content below:
Section "InputClass"
Identifier "touchpad"
Driver "synaptics"
MatchIsTouchpad "on"
Option "Tapping" "on"
Option "TapButton1" "1"
Option "TapButton2" "3" # Open context menu
Option "NaturalScrolling" "true"
EndSection