How to cancel the "Deutschland-Ticket" subscription?

How to cancel the "Deutschland-Ticket" subscription?

According to the legal framework for the "Deutschland-Ticket", cancellation up to the 10th of a month must be done, so that the subscription is terminated at the end of the month. The providers may have deviating deadlines.

DSW21 (Dortmunder Stadtwerke)

Info
With the provider DWS21, you can cancel your contract by the 20th of the previous month.
The termination can be made via the User Acount under item "Subscription overview" with DSW21. How can I manage the "Deutschland-Ticket" subscription with DSW21?