here is a way to create a shortcut on your desktop to lock the screen (or in the menu)

go to app manager and install scm (shortcut maker)(thanks to tiboric)

its name is scm yes "scm" only don't confused

now open shortcut maker(you have just installed)

tap to CMD and give a name to your shortcut
(don't check box "run in xterm")
and put this in command box (better to copy the code and paste it)

and prees create...

now there should be a new icon in the menu to lock Ur screen

now go to desktop > settings > add your shortcut DONE........ !