I can think of different ways to get it to work, but none that will take effect until you exit the list box.
The change event "does not initiate in response to changes in the object values as a result of calculations or scripts, or by the merging of the form design with data."
With the click event "the value of the field does not change in response to calculations and scripts until the field loses focus."
These are quotes taken from the help section in Designer, you can find them by searching for Interactive events. You'll be able to change the value of your text field based on the selection but it won't take effect until the list box loses focus.