How to add a Flatpak's app to Gnome 3 auto-starts?

Auto start x applications:

One way to make an application auto start at user login is to add its .desktop file to /etc/xdg/autostart/ this will affect all users

Just copy or link (ln -s) your telegram.desktop to /etc/xdg/autostart/ if you don't have a telegram.desktop create a new one, here is an example with Deepin Music Flatpak's package

[Desktop Entry]
Comment=
Exec=flatpak run com.deepin.Music
GenericName=
Icon=deepin-music
MimeType=
Name=Deepin Music
Path=
StartupNotify=true
Terminal=false
Categories=Music;
Type=Application
X-DBUS-ServiceName=
X-DBUS-StartupType=

Starting from the answer given by @intika I found a solution I like more.

Instead of replicating the content of the existing desktop-file in /var/lib/flatpak/exports/share/applications/org.telegram.desktop.desktop I linked it inside my personal ~/.config/autostart/. Works like a charm :-)