The app is installed in
/opt/telegram/
and the binary is
/opt/telegram/Telegram
To start the messenger via command line, you have to use the full path or add the path to your PATH definition or create a symbolic link in /usr/bin:
Use the PATH
Edit the file ~/.profile
nano ~/.profile
and add the line below at the end of the file
export PATH="$PATH:/opt/telegram"
OR
You have a desktop file:
/usr/share/applications/telegram.desktop
and therefore use the command below
cp /usr/share/applications/telegram.desktop ~/.config/autostart
to create an autostart entry via command line.

Why do I know that?
% dpkg -L telegram
/.
/usr
/usr/share
/usr/share/doc
/usr/share/doc/telegram
/usr/share/doc/telegram/copyright
/usr/share/doc/telegram/changelog.Debian.gz
/usr/share/applications
/usr/share/applications/telegram.desktop
/opt
/opt/telegram
/opt/telegram/telegram.tar.xz
/opt/telegram/telegram.svg
% cat /usr/share/applications/telegram.desktop
[Desktop Entry]
Version=1.0
Type=Application
Terminal=false
Name=Telegram
Exec=/opt/telegram/Telegram
Comment=Network taking back our right to privacy.
Icon=/opt/telegram/telegram.svg
Categories=GNOME;GTK;Network;