Hi,
You need to protect the SPS admin UI and you should have configured the domain, auth scheme, policy, user directory.
The steps to protect the SPS admin UI can be found from
CA SiteMinder Secure Proxy Server Administration Guide Release 12.5 -> Chapter 2: Install, Upgrade, and Configure the SPS -> Protect the Administrative User Interface
The user that can login to SPS admin UI is from the user that you configure in policy.
If you have configured user1 in policy, the credentials od user1 is what you need to login to the SPS admin UI.