test.js

function addComboFormField() {
	TXTextControl.formFields.getCanAdd(canAdd => {
		if (canAdd) {
			// Add form field
			TXTextControl.formFields.addSelectionFormField(emptyWidth, ff => {
				var cars = ["Porsche", "BMW", "Dodge", "Mercedes", "Ford"];
				ff.setItems(cars);
				ff.setSelectedIndex(4);

				highlight(ff);
			});
		} else {
			console.log("Cannot add field at that location.");
		}
	});
}

function highlight(formField) {
	var highlightColor = "rgba(9, 165, 2, 0.3)"; 
	var highlightMode = TXTextControl.HighlightMode.Always;

	formField.setHighlightColor(highlightColor);
	formField.setHighlightMode(highlightMode);
}