
Hello @Adrian Teuscher ,
you can add your new custom domain to your Microsoft 365 tenant at any time. From that point on, the custom domain is assigned and reserved to your tenant. When adding the domain, it must be an onmicrosoft.com domain. For example, if you want to rename your tenant to "fabrikam.sharepoint.com", the domain entered should be "fabrikam.onmicrosoft.com". You do not need to purchase the "onmicrosoft.com" domain to add it, and no public DNS registration is required.
Check the availability of the new domain you want. For example, if you want your SharePoint and OneDrive URLs to start with fabrikam.sharepoint.com, type https://fabrikam.sharepoint.com into a browser. If you get a message that the address could not be found (404), it is probably available. If you get a login screen or a message that your username could not be found in the fabrikam.sharepoint.com directory, the domain is already in use and you need to try another domain. If the domain is already registered to another customer, we cannot provide you with any information and cannot contact the customer.
If you own the domain for another subscription, you must delete that tenant in Azure AD. Deleting a tenant usually takes three days for the domain to become available.
DO NOT use the domain to test this procedure in a test environment first. If you do, you will not be able to use the domain for your production environment.
If this is helpful, please accept the answer. Thank you.