VBScript Set a property that isnt on a class

I want to use the BeforeCreateNewObjectFinalize Event Handler to setup a Empty property that isnt on a certain class.

Here is a example how i did it:

Option Explicit

Const I_OBJTYPE = 0

If ObjVer.Type = I_OBJTYPE Then

PropertyValues.SearchForProperty(1300).TypedValue.SetValue MFDatatypeText, ""
Vault.ObjectPropertyOperations.SetAllProperties ObjVer, true, PropertyValues

End If

This script works fine if the property is already on the class itself while creating. But the main functionality should be here to set the property 1300 on the class and set the empty value. Like i would use "Set Properties" on the WF State, but that isnt a solution because it sets a empty value everytime i revide the document.