Launch an App by key press or a combination of key press - Window phone 8.1, iphone, Android

I am planning to build an app which needs a screenshot. My requirement is to launch the app when the designated key presses are executed. (i.e. like volume up+lock key for taking screen shot in windows phone). My app will then launch automatically for further processing of that image. Is it possible to build or is there any work around for that?

Thanks in advance.