This method is called when an HTTP authorization implementation requires a user name to access a protected URL.
A pointer to a buffer where the IAuthInfo implementation is to store the required username.
The address of a DWORD that, on entry, contains the size of the buffer provided as szUid. Implementations of IAuthInfo should not exceed the buffer size when providing domain information. On exit, pdwBuffSize should contain the size of the username including the NULL terminator.
The GetUsername method is typically called by a CAtlBaseAuthObject derived class that is attempting to satisfy the security requirements of a requested URL.