http://windows.microsoft.com/en-gb/windows-10/keyboard-shortcuts-for-game-bar
Keyboard shortcuts for Game bar
Applies to Windows 10
When you’re playing a game on your PC, here are shortcuts that you can use to record clips and screenshots.
Windows logo key + G: Open the Game bar
Windows logo key + Alt + G: Record the last 30 seconds (you can change the amount of time recorded in Game bar > Settings)
Windows logo key + Alt + R: Start/stop recording
Windows logo key + Alt + Print Screen: Take a screenshot of your game
Windows logo key + Alt + T: Show/hide recording timer
You also have the option of adding your own shortcuts. To do that, open the Xbox app and go to Settings > Game DVR > Keyboard shortcuts.