You do not need to have the HFC in order to buy tickets with reduction. You can just mark “half fare card” under reductions when purchasing.
The you have the card that entitles you to travel at half price is verified by the conductor on the train, so in theory you can even buy half price tickets now, and then buy the card later, eg. at the train station.
For Geneva - Zermatt look also at supersaver tickets. We normally suggest people buy a day saver pass if they want to leave on the first train after they get of the plane, as that is flexible. But since you plan on doing some sightseeing in Geneva you can also buy a supersaver ticket.
So just go to www.sbb.ch, enter the date and time you want to travel, and the system will propose the cheapest option for you. Of course. booked in advance the cheapest option offered is often the day pass, which in your case you can then indeed just buy.