How do I manually control the Lock screen activation?

I want the lock screen function to activate immediately after pressing the power button (prefered) or other physical button, but only if I press that button. I don't want the lock screen to activate under any other circumstance, even if the display goes to screen/power saver mode. I know that I can set the time that it takes to activate the lock screen after the screen saver activates, but there is no "infinite" time among the options provided. Anyways that's not what I want, again to summarize I press the power button (or any other physical button), the lock screen activates instantly, I don't press the power button, lock screen never activates.

How can I do that?

Unit model and OS: LG Optimus L90 d415e Android 4.4.2 Kitkat

Regards.