How do I define the choices for the item type 'Message Choice'?

I need to add a check box to a page in iProcurement.  We want to force the user to acknowledge a message before they submit a requisition.  In my OA framework personalization testing, I determined that the 'required' attribute for the item type 'message check box' does not work, so I can't use that.  I decided to use the item type 'Message Choice'.  I want the only choice in the drop down box to be 'Y'.  Where/How do I define what the choices are for this item type?