IF you are asked to go through the identiy check this means that you have somehow gotten misdirected to the page where you purchase the domestic Half Fare Travel Card.
Fore the Half Fare Card for tourists this ID check is not needed.
Read this:
https://community.sbb-tickets.cc/blog/35831-swiss-half-fare-card-how-to-buy-and-details
Alternatively buy it here:
www.swissrailways.com
Or buy it at the station here.
You do not need to buy tickets in advance, but if you still wish to do so you can indicate that you (will) have the Half Fare Card when purchasing. It is on the train itself that your HFC will be checked.