How to set external monitor as primary monitor

1. Open Terminal and run xrandr command
2. Make a startup application file
mkdir -p ~/.config/autostart
pluma ~/.config/autostart/primarymonitor.desktop
... the contents:
[Desktop Entry]
Comment=Set the default monitor
Exec= xrandr --output VGA1 --primary

… save and quit

PS. Use your own xrandr info for primarymonitor.desktop file.

The command seen the right way xrandr_output