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Here is a message box that you can display from service applications. Change MB_OK to alter the buttons that appear. If you want the return value from MessageBoxEx change PopServiceBox into a function.

    procedure PopServiceBox(Title, Text: string);
    begin
      MessageBoxEx(
        0, PChar(Text), PChar(Title),
        MB_OK or MB_TOPMOST or MB_SERVICE_NOTIFICATION, 0
      );
    end;