Create powerbutton

https://alexcabal.com/turn-your-laptop-screen-off-with-a-keyboard-shortcut-in-ubuntu-karmic/

key code 124 (the power button) to map to F15
Then edit 

File: ~/.config/openbox/lubuntu-rc.xml

<keybind key="F15">
  <action name="Execute"><execute>/usr/bin/powerbutton</execute></action>
</keybind>