Hi,
There's no support for that in the GUI (but you can use a hack, see below), from either install scripts or manually you have to add your statement at the end of the shortcut script (that resides in ~/.PlayOnLinux/shortcuts/)
echo 'xrandr -s 0' >> ~/.PlayOnLinux/shortcuts/'Shortcut name'
If the game starts asynchronously you may have to add a "wineserver -w" statement before your xrandr statement.
But if you insist in using the GUI, it can be done with a hack:
Configure button > (select shortcut) > Miscellaneous tab > Commands to run before the program:
trap 'xrandr -s 0' EXIT
Which will run your statement on shortcut exit, achieving the same thing