If the realm to be used to authenticate the user is the one specified as the Default Realm, in the GridGuard Radius setup, the username can be entered simply as the username itself; for e.g., johndoe

If a separate realm is to be used to authenticate the user, then the username needs to include the realm name as part of it. The username can either be specified as 'myrealm\johndoe' or 'johndoe@myrealm'.