Cinnamon 3.8 introduced one more bug. It is not possible now to add autostarted apps thru System Settings --> Startup Applications --> Add --> Custom command. Cinnamon simulates the usual process of adding a new program, but when the Add button is clicked at the end, nothing happens. A program is not added to the list of autostarted apps.

While waiting for a fix, it is possible to use a simple workaround. Cinnamon’s Startup Applications tool does nothing but copies a .desktop file into the ~/.config/autostart folder. When a startup app is deleted from the list, the tool simply removes its .desktop file from the ~/.config/autostart folder. Removing with Startup Applications tool works.

It is possible to do the same by hand, without Startup Applications tool. To add a new startup app, simply copy or link its .desktop file into the ~/.config/autostart folder. To remove a program from the list of autostarted apps, delete its .desktop file from this folder.

Mate, Kde, Gnome use exactly the same mechnism. Autostarted apps may be added and removed by hand.