It is possible to buy it online but I don't think there is enough time (production time and abroad shipping).
The easiest way is to buy the half-fare travelcard directly upon arrival at the SBB ticket office at Zurich Airport. It is important to provide a fixed address in Switzerland for the delivery of the card. Your first and last name must be on the mailbox.
Required documents: For the Half-Fare travelcard you have to bring the completed order form, an original passport photo and your passport or ID card to the ticket office. I advise you to print the order form, fill it out and bring it to the SBB counter.
You will find more information and the order form here.
I wish you a good afternoon and for the next month, a wonderful stay in our country.