My way to do it. Install Menulibre and make an entry:
sh -c "dbus-send --session --type=method_call --dest=org.buddiesofbudgie.PowerDialog /org/buddiesofbudgie/PowerDialog org.buddiesofbudgie.PowerDialog.Toggle"
Save it and drag the app into Plank.
I'm going to write this into "Fun with Plank" thread.