Consegui atingir meu objetivo usando esse código de esqueleto:
Private Sub Document_New()
Dim strValue As String
strValue = InputBox("Enter a value for 'myproperty':", "myproperty", " ")
' the value will be an empty string, "", if the user cancels
' or deletes the default space; fix that
If strValue = "" Then strValue = " "
ActiveDocument.CustomDocumentProperties("myproperty").Value = strValue
ActiveDocument.Fields.Update
End Sub