Get current user from app running as a service under the system account

i have an app that runs as a service and under the system account and i need it to get the current logged in user, if a user is logged in, but with the following code all i get is the system account. any way to get the actual logged in user if there is a logged in user?

Code:

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Private Declare Function GetUserName Lib "advapi32" _
  Alias "GetUserNameA" _
  (ByVal lpBuffer As String, _
  nSize As Long) As Long



Private Function GetThreadUserName() As String

  'Retrieves the user name of the current
  'thread. This is the name of the user
  'currently logged onto the system. If
  'the current thread is impersonating
  'another client, GetUserName returns
  'the user name of the client that the
  'thread is impersonating.
  Dim buff As String
  Dim nSize As Long

  buff = Space$(MAX_USERNAME)
  nSize = Len(buff)
 


  If GetUserName(buff, nSize) = 1 Then

      GetThreadUserName = TrimNull(buff)
     
      Exit Function

  End If

End Function

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