Hi,
The syntax for JavaScript in the preOpen event of the dropdown would be:
this.addItem(textfield1.rawValue);
But you would also want to include script to clear the items first, otherwise the items on the dropdown would multiply everytime it is opened:
this.rawValue = null;
this.clearItems();
this.addItem(textfield1.rawValue);
this.addItem(textfield2.rawValue);
You can see this type of script in action here: http://assure.ly/jcTahK and here: http://assure.ly/gcXx03.
Hope that helps,
Niall