Hello, @Mario Meirhaeghe,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you want to know how to add your own account as an M365 administrator if your company does not have an active M365 administrator.
By default, for safety, if your organization does not currently have an administrator, it is not possible to add a new administrator properly. If your company's previous administrator account still exists, please try to retrieve the previous administrator's account. After logging in to M365 with this account, you can set your account as administrator by following the steps below.
If the previous administrator account has been deleted or cannot be found, it is recommended that you contact the Microsoft support team via phone or online chat. They will guide you through the process of restoring access and help you assign the necessary administrator roles to your account. For information on how to contact support, please refer to Get support - Microsoft 365 admin | Microsoft Learn.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang