How do I change the Umi Super's shortcut button to take screenshots? I have a Umi Super (rooted) device with one shortcut button (press it, and it opens an app of your choice)

I want to remap the key to take screenshots.

I have tried Button Mapper - but it takes to much time to take the screen shot, or sometimes it does not even take it.
I am open to any solutions, exposed modules /root apps / etc
Install Xposed edge module (see the help and support section on basics of how to use this module) 
Select Keys from main menu and tap to reveal the next screen which has hardware keys listed (You can also add hardware keys if not listed). In the corresponding drop down menu you have actions of click, double click and long press. Map the action to taking screenshot (full screen / selected region)