Private Sub txt1_TextChanged(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles txt1.TextChanged
        If (txt1.TextLength > 5) Then
            If Not txt1.Text.Chars(5) = "*" Then
                txt1.Text = txt1.Text.Insert(5, "*")
                txt1.SelectionStart = txt1.TextLength
            End If
        End If
    End Sub