This works for me:
XmlDocument.DocumentElement.SetAttribute("xmlns:xsi", "http://www.w3.org/2001/XMLSchema-instance");
XmlDocument.DocumentElement.SetAttribute("xmlns:xsd", "http://www.w3.org/2001/XMLSchema");
If you want to create the entire document you've posted, you might not want to forget the XML declaration:
XmlDeclaration xml_declaration;
xml_declaration = XmlDocument.CreateXmlDeclaration("1.0", "ISO-8859-1", "yes");
XmlElement document_element = XmlDocument.DocumentElement;
XmlDocument.InsertBefore(xml_declaration, document_element);
In certain cases you might need it.
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