E.g. If you want to add a 40 day through on September 5th, 2018 you just add a line and put in a date. Like this:
plotshape(time == timestamp(2018,10,5,0,0) ? 60 : na, color=blue, style=shape.diamond, text="", location=location.absolute, size=size.small)
The last line (it is two lines for 20 and 40 day cycles) adds a future through based on Hursts nominal model. You need to update the dates for the calculation manually.
I released this script on request. Realistically it is very bad because I couldn't solve a lot of problems with Pine script. Why did the script get 11 likes I have no idea...