The SetFocus function sets the keyboard focus to the specified window. The window must be attached to the calling thread's message queue.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winui/winui/windowsuserinterface/userinput/keyboardinput/keyboardinputreference/keyboardinputfunctions/setfocus.asp

DECLARE INTEGER SetFocus IN user32 AS SetFocusAPI INTEGER hWindow
? SetFocusAPI(_screen.HWnd)

If the function succeeds, the return value is the handle to the window that previously had the keyboard focus.