Set ShapeTool property as SpRectangle,then you can draw it by mouse.

Also,you can draw it by code.

Code example:

procedure TMainFrm.Button1Click(Sender: TObject); 
begin 
  if MyCAD1.GetShapeByName('RectangleShape') = nil then 
    MyCAD1.AddShapeByCode(MyCAD1,spRectangle, 'RectangleShape', [MyPoint(290, 211),MyPoint(380, 211), MyPoint(380, 270), MyPoint(290, 270)], 30 * pi / 180);
end;