I did not find such information on the Paymill website.
I have got an answer from Paymill:
Our descriptor normally contains your URL or company name. Depending on the acquirer there might be other information be provided as well, like your phone number or the related reference number of the acqurier transaction.
A fully dynamic descriptor we do not offer at the moment. If you have the feeling that the provided descriptor is not sufficient then please do not hesitate to advise us on your preferreddescriptor and will review the possibilities together with the acquiring banks.