[xmonad] using rdesktop with xmonad

hi, i like to use rdesktop -f xxx.xxx.xxx.xxx to connect to my windows pc, in fullscreen mode. but after entering the rdesktop window, i can not switch out anymore, M-1/2/3... can not switch workspaces anymore. i have to disconnect the remote machine to switch out. how can i use xmonad's hotkey inside a rdesktop window? thanks lars