Need to determine Autolog Username

[josborn]

I don't think you can do this, you might consider a different approach.
This post has an alternate way of doing what i think you want.

Enable or Disable Auto-Login according to OSUsername

Using an Active Directory user source we’ve used Enable SSO Login (bottom of picture). Rolls are handled in Active Directory Groups. Rolls that should be allowed to launch the project are put in the Required Client Rolls (Project Properties → General → Required Client Rolls). Our production stations automatically log in because their user is a member of one of these rolls. Desktop users who are authorized are automatically logged in because they are part of a different, but still authorized roll…