Well, if you have already submitted your move, and they haven't viewed the game yet, you can go back to it and click "Edit" near the top, and add a message. If they have already viewed the game (but not yet played) you cannot do that.
(verberg) Houdt uw Postbus schoon door belangrijke berichten te archiveren en regelmatig de Verwijder-alle-berichtenknop in uw instellingen. (pauloaguia) (laat alle tips zien)