I don't think you can add vertical reference lines as you want them.
I have come up with another solution which i think looks best using markers.
You could convert it to a combo chart and add an extra measure that is an if statement.
Note - I used week numbers for my calculation and this could be converted to actual dates
Main expression = Sum(Sales)
Extra expression = if(Week= 6 or Week=10, sum(Sales))
Display the Extra expression as a Marker (could use bars but personally don't think it looks so good) and make sure colours are different.
Result would be like below.
Hope this helps