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?
The following post should answer your question: Why do I need three server license?
Basically in your case, you probably received from your Tableau Salesperson 2 server license keys: one that will give you the ability to add 7 explorers in your server, and one that will give 2 creators. You should thus add all server license keys, in Tableau services manager ( Sign in to Tableau Services Manager Web UI - Tableau , and View Server Licenses - Tableau ).
Please note that those keys are server keys, and not desktop keys. So for your 2 developpers, you will probably also need additional keys for Desktop softwares (1 tableau desktop key per developer, and potentially 1 tableau prep desktop key per developer) .