After you created the TFS Hosting account, the system will create a default Admin account for you ([org ID]_Admin). If you do not want to use this user name for your admin account, you can follow the steps below:
1. Log into thehosted portal, and go to TFS Hosted -> User Management.
2. Locate the user (105427_Admin in this example), and click Delete button to remove this default user.
3. Now you can create a new user and grant the admin right to it. For example, I will use 105427_Robby as the admin account, so I just need to add this user to the pre-defined Administrators groups.
To do that, please refer to these articles: