text fields is populated by a dynamically populated drop-down

You'll need to script the drop down list's Change event. In the Change event, you can access the xfa.event.newText property which which will contain the text value of the item the user selected from the list. If that's what you want in the text field, then simply assign the text value to the text field:

myField.rawValue = xfa.event.newText;

If you want the value associated with the text that was selected, you can use the boundItem method:

myField.rawValue = this.boundItem(xfa.event.newText);

If your list items have text and value data, the above statement will assign the value data (associated with the selected text data) to the text field.