Which service is the Swisscom Benefits voucher for?
go to the corresponding service and use the voucher
if in doubt, contact the Swisscom Hotline
Private customers: contact by hotline, chat, message | Swisscom
“On apprend parfois plus d'une défaite que d'une victoire” — José Raúl Capablanca
As the link provided by @“Black Mamba” is written in French, here is the link in your language, German: https://www.swisscom.ch/de/privatkunden/hilfe/kontakt.html
Doremi