Don't know if that is what you are searching for, but in menu Edit, Preferences, General tab, there is a place to put your default user name, that is not related to the current open file but to your filemaker setup for this machine.
In File options, you are not specifying a user name. Instead you are specifying a default account name. Account names are specified in Manage | Security (FMP 11) or Manage | Accounts & Privileges (older versions). User names are entered in preferences and are specific to a given OS user account on the computer that launched filemaker.
Filemaker confuses the issue by automatically entering the user name as the default account name--an option I'd prefer to be able to turn on/off for a given file as this default is not always appropriate in every situation.