On Office 365, It’s not obvious how to change your own Username Default Domain Name from let say @mydomain.onmicrosoft.com to something like @mydomain.com. Thus if you have only 1 license. Well, I found it out.

1. Create a new User Account and provide it with Administrative Rights. You won’t be able to assign Outlook, Lync or SharePoint licenses (in my case I have none), but this account still has the administrative rights
2. Log out from your default account, and log in with your new Admin Account.
3. Go to the Admin –> Users section
4. Edit your default account, and click on Properties
5. Change the Default onmicrosoft.com domain and assign your own domain.
6. Log out and then log in with your default username but instead log in with @mydomain.com.