uses
  OXmlPDOM;

procedure TForm1.SubmitButtonClick(Sender: TObject);
var
  XMLNode: PXMLNode;
  XMLDocument: IXMLDocument;
begin
  XMLDocument := CreateXMLDoc;
  XMLDocument.LoadFromFile('C:\File.xml');

  if XMLDocument.DocumentElement.SelectNode('//categories', XMLNode) then
  begin
    XMLNode := XMLNode.AddChild('cat1');
    XMLNode.Text := Edit1.Text;
    XMLDocument.SaveToFile('C:\File.xml');
  end;
end;