Hi Audrey,
You need to send Press F key action to ses[0]/wnd[0] as shown below:
For user key press event as Tamas Hoznek mentioned, it is browser dependent. If browser consumes F4 key (like in IE, it opens address bar dropdown) then the event is not passed to Silverlight. So as an alternative we have provided Shift+F key (Eg: Shift+F4) which you can use.
Thanks
Chinthan