'/System/Library/CoreServices/Menu Extras/User.menu/Contents/Resources/CGSession' -suspend

• use “Automator” create a service, then set the input as “no input”.

• drag “Run shell script” to the editor in right side, then

    '/System/Library/CoreServices/Menu Extras/User.menu/Contents/Resources/CGSession' -suspend
  

• save the service as “lockscreen”

• set the shortcut key for the service in keyboard->shortcut keys