[xmonad] managehook: How to shift apps to a screen in dual-head?

Hi all! I got a proper dual-head xrandr configuration with one monitor in landscape view and one in portrait view. So in xmonad I have 2x10 workspaces, which is fine. Sure I can manually navigate and switch the focus or push clients to a specific monitor/screen by mod+{w,e} etc., no problem. – But how to initially shift programs to workspaces on a specific monitor (in myManageHook)? Cheers