Hi,
Thank you for posting in Symantec community.
To make a domain to a default domain Go to:
Admin-> Domain -> Select the Domain that you want to make as the Default Domain (Here making default domain refer to the domain that which will be the main domain for login)
From the Task pane Click on Administer Domain.
Doing so will make that domain as the main domain to Login to SEPM.
Screenshot is attached to the reference.