How to lock screen with keyboard on Linux Mint (LXDE)

In Linux Mint 18.3 (Sylvia) GUI, go to Menu -> Preferences -> Keyboard -> Shortcuts.

Expand the System tab and you should see a Lock Screen shortcut setting.

The default shortcut to lock the screen is Ctrl+Alt+L.

I use mint10 where you go to menu-> preferences -> keyboard shortcuts.
I have not been able to find any gui lxde and do not have away of testing these methods.
LXDE:Questions,
Search to Index 3. mouse/keyboard,for mint 11 the file is mintlxde-rc.xml.
From Stray notes,
From Sourceforge,
From openbox/org,
I hope something helps.

Add the following to the <keyboard> section of ~/.config/openbox/lxde-rc.xml :

keybind key="C-A-l">
  <action name="Execute">
    <command>xscreensaver-command -lock</command>
  </action>
</keybind>

Then run this command:

openbox --reconfigure

Now Ctrl-Alt-l should lock the screen.