Datasheet

Just add the following lines into your InputClass of /etc/X11/xorg.conf.d/99-calibration.conf after the calibration section
Option "EmulateThirdButton" "1"
Option "EmulateThirdButtonTimeout" "750"
Option "EmulateThirdButtonMoveThreshold" "30"
So for example your file will look like:
Section "InputClass"
Identifier "calibration"
MatchProduct "stmpe-ts"
Option "Calibration" "3800 120 200 3900"
Option "SwapAxes" "1"
Option "EmulateThirdButton" "1"
Option "EmulateThirdButtonTimeout" "750"
Option "EmulateThirdButtonMoveThreshold" "30"
EndSection
This makes a right mouse click emulated when holding down the stylus for 750 ms.
(Thx adamaddin! (https://adafru.it/fH3))
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