Creating "real" dropdowns for a GUI prototype

[Mark]

I've been attempting to create a "real" dropdown for a GUI prototype where if
I select one of the Action.Row values (either using the right-mouse click or
custom properties), I want the dropdown text to fill in with the selected
value.

Currently, I am able to populate the Custom Properties - Value and also
display it with a mouse-over (or Miscellaneous - Comment field), but I cannot
figure out how to have this dynamically display within the text of the
dropdown field.

Any suggestions???