Create drowpdown from filled in list

It's a bit tricky. The list will have to be updated each time the text fields are edited, which will remove the user's current choice in it. So I would recommend you use a button to update the list, instead.

Let's say you have 4 text fields called Text1 to Text4 and the drop-down is called Dropdown1.

You can use the following code as the MouseUp event of said button field:

var items = [""];
for (var i=1; i<=4; i++) {
	var v = this.getField("Text"+i).valueAsString;
	if (v!="") items.push(v);
}
this.getField("Dropdown1").setItems(items);