After you have registered a domain name, you need to add at least one user with a Phone System license (in my case I am using an E5 license which includes Phone System) and the SIP address of that user should have the FQDN portion of the SIP address matching the created base domain. This is needed to complete the registration of the SBC.
On Microsoft 365 admin center, expand Users > Active users
Click on Add a user
The “Add a user” wizard will be started
Fill in the details of that user, the most important part is that under “Domain” you make sure that you have selected the domain which will be used for registering the SBC
I cleared “Automatically create a password” this allows me to enter the password myself (that is just my personal preference)
I typed the password
I cleared “Require this user to change their password when they first sign in” (that is also my personal preference).
I typed the password and clicked on Next
The user needs to have a Phone System license. In my trial tenant, I have “Office 365 E5”, so I am assigning this type of license to this user
Click on Next
This will take you to Review and finish the page. Where you can check the settings that you have selected
Click on “Finish adding”
The wizard will work on adding the user
The wizard will confirm that the user is added
Click on Close
Now you will be able to see the user in the list of “Active users”. You might need to search the user if you have a big number of users
