Do an admin takeover in Office 365
Check the Domains FAQ if you don't find what you're looking for.
If you are an admin and want to take over an unmanaged or "shadow" tenant create by users who used self-service sign-up, you can do this with an internal admin takeover.
Step 1: Verify your email address
Create a user context in the unmanaged tenant through signing up with such as Power BI. For convenience of example, these steps assume that path.
Open the Power BI site and select Start Free. Enter a user account that uses the domain name for the organization; for example,
firstname.lastname@example.org. After you enter in the verification code, check your email for the confirmation code.
Once you receive your email, Sign out of Office 365 from your current account.
Follow the email link that shows your email address and a Yes, that's me button.
Step 2: Create a new account
When you follow the Yes, that's me link in your email, you'll be brought to a page where you create a new account.
Fill in the user name and password field with what you want to use once you're done and click Start.
Step 3: Verify domain ownership and become the admin
A wizard will open called "Become the admin". If the wizard doesn't start for you look for the Admin tile and click on it.
Click "Yes, I want to be the admin"
You'll need to verify that you own the domain you want to take over by adding a TXT record to your domain registrar.
The wizard will show you the TXT record to add as well as provide a link to your registrars website and a link to step-by-step instructions.
Once you've added the TXT record at your registrars site, come back to the wizard and click Okay, I've added the record.
If someone in the account signed up to use Office Online at work (instead of one of the other subscriptions, like Power BI), you'll be prompted to and required to buy licenses for them as part of becoming the admin for the domain. But you can add or remove licenses after you finish admin setup.