Debian / MATE / Gnome - Lock screen from terminal / on login

Add one of the matching following to the startup programs list somewhere in the GUI:

For Cinnamon:

cinnamon-screensaver-command -l

For Gnome:

gnome-screensaver-command -l

For MATE Desktop, run this:

mate-screensaver-command -l

You may also need to start the screensaver before it, using cinnamon-screensaver; or gnome-screensaver; or mate-screensaver;