Ok, just looked at it now. Unbelievable as it may seem, I think you are right - you can't add a custom command through the GUI. (Who thought this would be a good idea - I am wondering...)
But there seems to be a workaround. (You'll have to get your hands a bit dirty on the console though).
I created a starter for one of the proposed applications just to see what happens and found that a .desktop file was added to ~/.config/autostart
(gimp.desktop in my case because I added gimp as a startup program in my test)
In other words, if you list the entries of that folder
ls -l ~/.config/autostart
you'll see all existing startup entries.
Now you can create a .desktop file yourself and place it into that directory, and that should execute the configured command at startup.
Here is an explanation on how to write such files from the arch wiki
There are plenty of other resources on the web. Throw the term "how to write a .desktop file" at your favorite search engine.
Or, again, you might just switch to a leaner desktop environment :-) On openbox, you just add your startup commands to a script located at ~/.config/openbox/autostart and that's it...
Either way - best of luck!