Glad you like Endless! There is indeed a way to automatically start an app after login, but it’s a bit hidden and low level I’m afraid.
If you use the terminal to copy the .desktop file of the app you want to the ~/.config/autostart directory within your home directory (you’ll have to create the autostart directory yourself, but once it’s there everything works fine). You find the desktop files in /var/lib/flatpak/exports/share/applications/. So you’d type something like:
You could also create a link with the ln command but I always get the ordering of that messed up so I won’t suggest it in case I get the example wrong! I’m aware that this is a bit lame, sorry, but it does at least get you what you need. I’m hoping that we can add the GNOME Tweaks tool to the App Center at some point soon - that will give you a GUI way to manage automatically starting apps.
Inside a hidden directory called .config in your home directory. Open the Documents app and make sure you’re in the home directory, press ctrl+l to enter the location bar and add .config to the end - press enter and you’ll be there.