An action cannot act directly on a text box, other than via a macro. But you could create a variable that is displayed in the text box (='$(vVar)'). Create a field event "On Selection" trigger (Document Properties | Triggers) for the field in the list box with a set variable action.
The set variable action expression will determine what will be displayed in the text box.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein
