I am trying to go from Zermatt to Milan in July. I have the Swiss Travel Pass. have tried booking this from www.sbb.ch to go via Brig. I have heard Swiss Travel Pass covers you until Domodossola so if I book on sbb it should automatically decrease the fare accordingly. However when booking there is no option to add the Swiss Travel Pass - only an option to apply a Half Fare Pass.
How do I book this trip from Zermatt to Milan with Swiss Travel Pass?