How to enable screen saver for login screen?
To the best of my knowledge, it's not possible to trigger the screensaver from the lock screen. However, you could consider triggering the screensaver instead of going to the lock screen, then changing your security settings to lock your device in the background as soon as the screensaver triggers.

To do this, go to your Security &amp; Privacy preferences in System Preferences, check the checkbox next to Require password [time interval] after sleep or screen saver begins, and select immediately from the dropdown (or 5 seconds if you want a little leeway—this setting dictates the length of time after the screensaver starts before you'll need to put in your password to unlock your computer). See the screenshot below:



Next, you'll simply need to add a shortcut to start the screensaver to your Touch Bar. As this CNET article explains, go to System Preferences > Keyboard > Customize Control Strip and drag the Screen Saver control to your Touch Bar's Control Strip. From now on, instead of locking your display, press the Screen Saver icon on the Touch Bar to both lock your device (i.e., prevent access without a password, but without showing the login screen) and start the screensaver.