What you can try to do is use ComboBox as standard HTML Combo Box(Element Type) in Application Modeller. But in the Navigation step use Select Item as Action and in Inputs section use Item Position as a Number.
Where 0 is the first position in Combobox, blank. 1 is the first position, in your case, it would be "new customer" and 2 would be "old customer".
I think this'll work!