Thank you for your advice but I already found the answer to my question using Help of SODOCO. I haven't seen there is help link when you go to Parameters > About.
I intented to post an answer shortly.
If my message can help someone else, here is below the code for VBA :
'Allowed values : C_Unrestricted, C_Restricted, C_Confidential or C_Strictly_Confidential
ThisWorkbook.BuiltinDocumentProperties("Keywords").Value = "C_Unrestricted"
'Allowed values : Unrestricted, Restricted, Confidential or Strictly Confidential
ThisWorkbook.CustomDocumentProperties.Add Name:="Document Confidentiality", Type:=msoPropertyTypeString, LinkToContent:=False, Value:="Unrestricted"