I was able to create post-processing scripts on previous versions of Cura. However, with a Prusa I3 printer configured, the scripts do not write to the g-code file using Cura 3.6.
I tried deleting configurations and reinstalling, but it did not help. I am running a Monoprice Maker Select V2 printer. I tried setting the printer to Prusa i3 K2 as well.