Hi @Koen van Dijk,
Thank you for posting in this community.
The easiest way to start your developer journey with Microsoft 365 is to join the Microsoft 365 Developer Program.
Joining the program is free and by becoming a member, you get a free Microsoft 365 Developer Tenant, a free-instant sandbox that comes with sample user licenses, sample content packs for Microsoft Teams, email and calendar and is pre-configured for development, so you can get started quickly.
The tenant will renew every 90 days, so if you keep using it, it will be yours forever.
If you have problems with the subscription process, look for solutions from this article: Microsoft 365 Developer Program FAQ.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.