Most municipal discounts are granted automatically without a certificate. However, some municipalities are exceptions to the rule and explicitly ask for the discount to only be granted upon presentation of a certificate.
In this case you cannot buy your travel pass directly online and you must inquire with the municipal authorities as to how you can buy such a travel pass.
Did you receive a certificate from the municipal authorities? Then you can submit an online application for a travel pass with a municipal discount. Complete the contact form and upload the certificate. The Travel Pass Department will send you an email with payment instructions.