Hello Sab2
Halbtags means half day, while you probably mean Halbtax - half fare 🙂
Such vouchers do not exist for free, they come with strings attached as part of marketing campaigns. And, they can only be redeemed by people living in Switzerland or in one of its neighbouring countries, not by other foreigners.
The regular Swiss Half Fare Card destined to tourists is available here:
https://www.sbb.ch/en/tickets-offers/tickets/guests-abroad/swiss-halffare-card.html
Kind regards,
Roland