Runtime Variables in Dispatch Edit Box

Has anyone figured out how to enable the runtime variable pulldown in a Dispatch Edit Box? I am trying to use the edit box to assign a change number that will later be used to set a file variable. I have set up the variable but can't figure out how to assign the value entered into the edit box to it. The Variable Name setting in the edit box definition say "There are no run-time variables to show." and is grayed out. How do you create a run-time variable?