Hello Nico.
I think the information provided in this post is still valid.
If you are having issues, I would recommend to contact Autodesk Support to troubleshoot it.
About how to automate the process with the API, I could find this Restful API to list and assign licenses to users. I didn’t test it yet myself, but it may help in one direction. Unfortunately there is no unassign operation.
https://developers.shotgridsoftware.com/rest-api/#assign-subscriptions-to-users