I saw that the question posted about 1year ago in this community did not get answered to, so posting here.
First go to the Trenitalia website. Once you choose a train, you have the option of choosing the price bracket. Press see other offers, choose HTX Switzerland. The subscription number is the 6 digit code above your birthday on your SwissPass.
You may ask, why not just use the SBB website? Tranitalia lets you choose seats for the train between Milan and Switzerland, while the SBB site does not, even if you go to the station ticket counter, the people there do not have the ability to choose seats. I had to choose a seat to buy a seat next to my friend who had already bought tickets. Hope this helps someone.