You can use this module with the following in your ~/.xmonad/xmonad.hs:

 import XMonad.Prompt
 import XMonad.Prompt.Workspace

   , ((modMask x .|. shiftMask, xK_m     ), workspacePrompt defaultXPConfig (windows . W.shift))

For detailed instruction on editing the key binding see XMonad.Doc.Extending#Editing_key_bindings.