Tereza Hello. You can get the Monthly Dog Pass online or at many staffed public transport points of sale – it will be valid directly on your existing SwissPass.
If you do not already own a SwissPass, you can order one by phone via the SBB Contact Center on 0848 44 66 88 (CHF 0.08/min.) or at a staffed public transport point of sale. Important: please note that when ordering a SwissPass for the first time, you must have an up-to-date, high-quality passport photo.
More information: https://sbb-tickets.cc/en/tickets-offers/travelcards/dog-pass.html
I wish you a pleasant day.
Regards
SvenjaV