When you first sign up for Office 365 you need to add your domain name. This process also applies if you want to add an additional domain name to your Office 365 account.
This article shows you how to add a domain to Office 365 and verify the domain name using a TXT record.
1. Log into https://office.com using your Office 365 email account and password.
2. Click on 'Admin'. If you have not got the Admin option then you are not in the administrator group and you can not add domains.
6. The information needed for your TXT record is shown. Add a TXT record to your domain name. Instructions on how to do this can be found in this article Adding a TXT Record to Your Domain Name for Verification. Once the TXT record is set up click the 'Verify' button.