test.js

function addDropDownFormField(items) {

	TXTextControl.formFields.getCanAdd(canAdd => {
		if (canAdd) {

			TXTextControl.formFields.addSelectionFormField(0, sff => {
				
				// no custom text
				sff.setEditable(false);
				sff.setItems(items);

				// select "Uber"
				sff.setSelectedIndex(3);
			});
		}
	});
}

var transportation = ["Bicycle", "Car", "Scooter", "Uber", "Cab"];

addDropDownFormField(transportation);