Hello,
Do not use the built-in SEPM "admin" account when setting up Active Directory Authentication, doing so can prevent logon access to SEPM with "Authentication Failure" error. Lockout issues can occur when changing the Active Directory account, upgrading Active Directory, changing Active Directory mode, and when removing SEPM(s) as a replication partner.
SEPM Active Directory Authentication is only supported for Admin accounts that have been created in SEPM by clicking "Add Administrator."
NOTE: The SEPM user name is taken from SEPM database while the password is taken from Active Directory for the account you specified in Account Name.
Reference: http://www.symantec.com/docs/TECH104726
Once done check this Article as well:
Enable remember my user name & password option in Symantec Endpoint Protection Manager 12.1
https://www-secure.symantec.com/connect/articles/enable-save-username-password-option-121
Hope that helps!!