This build is not officially supported by Proton, please report issues at the package issue tracker.
To start automatically on login, create the file ~/.config/autostart/Proton Mail Desktop with the following contents:
[Desktop Entry]
Type=Application
Name=Proton Mail Bridge
Exec=flatpak run ch.protonmail.protonmail-bridge --no-window
X-GNOME-Autostart-enabled=true