How do I add a shortcut to the Show Applications menu in Ubuntu 17?

I was looking for a solution to do something similar - I installed an app from source and wanted to have it available in the "Show Applications" menu. Found a way to do that by running a grep for the name of one of the applications already shown in that menu.

So,

The obscure directory you're looking for would be: /usr/share/applications.

Just have a look at the *.desktop files there, choose one that doesn't have too much cruft, copy it to a new .desktop file and edit to reflect your app.

HTH...

One thing that I don't yet know how to do is how to have the "Add to Favorites" option available for it...

ps. once I know the solution, by rephrasing the question, I was able to find this thread that gives a few more solutions to a similar question: https://askubuntu.com/questions/79583/adding-custom-applications-to-gnome-launcher