Thank you for your answer Shivaram.
Unfortunate, it still does not solve my problem. I have already the creator license, but I don't know how to link that licens to the Tableau Server and thereby activating the extra seats.
Sorry Rasmus, I haven't yet worked on this new licensing model and my knowledge is limited. May be someone else can pick this up. Also, have you tried to reach out to your Account Manager or Tableau Support? May be open a case?