@Lodewijk Sioen Thanks for reaching out. I want to let you know that OSS is not a directly supported product, which means that we cannot guarantee support for it. I suggest that you submit a Github issue in the Open Source FHIR server repository https://github.com/microsoft/fhir-server, and we will do our best to address it as soon as we can. Thank you for your understanding.
For the managed service, we are making updates to the portal to make these same settings available in the Azure portal. That should be available sometime soon, but I don't have a specific ETA right now.
do let me know incase of further queries, I would be happy to assist you.