I remember fallout 4, alt + print screen keys to minimize. Can someone please let me know how to minimize The Witcher?

First, it is not a good idea to minimize full screen games. Best to use borderless window mode if you must switch back to the desktop. Minimizing any program in Windows is achieved easily by pressing the 'Windows Key', next to Left Ctrl. You can minimize all programs by pressing Windows Key + M .

Since the Windows Key doesn't always work, or almost never with newer games another way is Alt+Tab. That way you can choose which program/window you want displayed. It's what I use. But pause your game before you minimize in any of the mentioned methods (and save it, just to be sure) Some games don't like you minimizing them and crash when you try revisiting them.

Alt + tab does not work for me. I have not tried the Windows key.

UPDATE: Windows key brings up the desktop and game menu but does not minimize the game.
I have just installed an ultra wide monitor, resolution 2560 x 1080 and most of my screen saver screen shots
take up only half the screen width. I am hoping to re-produce them again at this higher resolution but to have to
exit the game every time I take a screen shot is going to be a real pain.

Win+Tab than. windows 8 and 10 has it as alt+tab. windows vista and 7 as win+tab. at least to my experience. you need to hold down either alt or win and then press tab multiple times until you reach the task you want displayed.