How can I launch screensaver with a keyboard shortcut?

I don't believe there is a built-in, system wide Keyboard Shortcut. The quickest way to activating the screensaver via the keyboard is either via a "Start Screensaver" command using a launching applications such as Quicksilver, Alfred, or Launchbar.

The quickest way of starting the screensaver using your mouse is via Hot Corners. Open System Preferences, click "Desktop & Screen Saver", click over to the "Screen Saver" tab and click the "Hot Corners" button in the bottom left. There you can specify a corner to roll over in order to start the screensaver.

If you want to start the screensaver quickly, because you want an easy way to lock your computer, a possible answer is located in the high-view "Mac OS X Hidden Features Tips & Tricks" question that remains active.

And specifically, this answer.

shift + control + eject send display only to sleep (great for locking your computer instantly)

Use Automator. Simply open automator and drag in the "start screen saver" action, select "no input" under the "services received" menu, and hit file > save as "Start Screen Saver". This will now appear under System preferences > Keyboard > Shortcuts > Services > General, where you can assign it any hotkey you desire, just as you would any other system action.

from http://osxdaily.com/2014/07/10/set-screen-saver-keyboard-shortcut-mac/