Map<String, String> args = HashMap<String, String>();
args.put("sclass", "myMessagebox");
Messagebox.show(
  "Do you want to return without saving?",
  "Confirmation",
  new Button[]{Button.OK, Button.ABORT, Button.CANCEL},
  new String[]{"Save and Return", "Abort and Return", "Cancel"},
  null,
  null,
  null,
  args
);