Option 1:
Use the PrintScreen, open paint, paste the screenshot and save the file.
Option 2:
You can try
NirCmd to send the WinKey + PrntScrn keystroke automatically.
nircmd.exe cmdwait 2000 sendkeypress rwin+printscreen
The above command simulates WinKey + PrntScrn after waiting for 2000 ms (2 seconds). Setting the timeout may be necessary if you want to capture a menu, or set an active window before taking a full screen capture.
You may create a desktop shortcut with the above target, and assign a hotkey as required.
Option 3:
How to Start Snipping Tool in Capture Mode by Default? » Winhelponline
http://www.winhelponline.com/blog/start-snipping-tool-capture-mode-default/
(To automatically select "Window" instead of Rectangle snip, change the code from:
nircmd sendkeypress alt+N r
to
nircmd sendkeypress alt+N w
See also:
How to Take a Screenshot in Windows? Different Ways Explained. 3rd party utilities like GreenShot and ShareX have many useful extra features.