Hello,
You need to use the InputBindings and KeyBinding classes, as following:
<window.inputbindings>
<keybinding command="ApplicationCommands.Open">
Gesture="CTRL+R" />
</keybinding></window.inputbindings>
Also, here's an article to help you out: http://stackoverflow.com/questions/1319425/application-level-shortcut-keys-in-wpf.