Hello Reeta
You must buy a ticket from the border station, in this case Basel Bad Bf. I suggest to buy it on bahn.de (German Railways) and to travel via Cologne (Köln). Between Cologne and Brussels, take an ICE train, not an Eurostar.
It is also possible to travel via Strasbourg (France) and a direct TGV from this city, in this case buy tickets from Basel SBB on sncf-connect.com (French Railways), but timetables are in my opinion less convenient and you could have a problem if you miss a connection due to a delay.
You will probably use a day of your pass to travel to Tirano, it is valid to this city in Italy. You can buy recommended and mandatory seat reservations on this site (it is possible to select “seat reservation only”):
https://www.rhb.ch/en/tickets/
Kind regards
Daniel