Hdv
You can‘t apply the voucher in advance, and also not online. Two months before expiration, you will automatically get an invoice on paper with QR code from SBB for the renewal. (along with a cancellation reminder in case you wouldn’t wish to extend the subscription, there will be at least another such reminder via email a few weeks later)
Now the point is that from home, like with any other received invoice, you could only pay the full invoice amount (170.-) via the usual bank transfer. For any other means of payment, such as credit card, cash, Reka or your voucher, you need to visit a SBB ticket sales counter at a major station instead. So you almost have two months time for that and can plan accordingly (and maybe set yourself some calendar entry?)