Hello Courraigie38
On trenitalia.com, the HFC reduction is only known for the direct trains from Italy to Switzerland, but not for internal connections, for example Brig - Zermatt. I don’t know if they have another software with more possibilities at the ticket office of Trenitalia in Milan or not.
It could also depend of the date of your travel, you have now replacement buses between Milan and Domodossola and sometimes also between Domodossola and Iselle.
Kind Regards
Danhiel