I'm not familiar with SAS Portal but a SAS stored process runs on a SAS Stored Process Server, and that server can be configured to run with different Multiuser Credentials.
On our SAS Server this is set to use a SAS service account - sas_srv. This may be different in your configuration so you will need to check in SAS Management Console to confirm this. For Windows authentification to work the user account(s) defined for the Multiuser Credentials must be defined in SQL Database as Windows Authenticated users.