- Log into your Partner Portal
- Click on Customer Accounts then Active Accounts
- Select your Customer by clicking Login next to their name
- Click on Services Menu then under Utility choose Domains
- Type the domain the Domain Name field and click Add
If you would like to set this as the primary address, tick the Default radio button next to the domain and click Save Changes. Please note that this will change all the existing mailboxes to have that as their primary and may require them to enter their credentials again in Outlook.