With Workbook.Sheets[1] do begin
  With Range['B2', 'B2'].AddComment('Comment text') do begin
    Shape.Line.Style := msoLineSingle;
    Shape.Line.DashStyle := msoLineDash;
    Shape.Line.Weight    := 0.25;
    Shape.Line.ForeColor.RGB := $0000FF;
  end;
end;