$property = "FooBar";
 $doc->CustomDocumentProperties($property)->{Value} = $text_value;

##</code><code>##

'If customdocument property did not exist, create it
    If Err.Number <> 0 Then
        Set DocProps = ThisWorkbook.CustomDocumentProperties
        DocProps.Add Name:="FooBar", _
                LinkToContent:=False, _
                Type:=msoPropertyTypeString, _
                Value:=str_content
    End If