You can set this either by using the Xfce Session Settings GUI interface (xfce4-session-settings), or by putting a "<name>.desktop" file in the following directory (create it if needed):
Code:
~/.config/autostart/
The file must include the
Exec key, followed by the command you want to launch, roughly as if you would do in a command line interface (you may need to escape or quote reserved characters).
Code:
[Desktop Entry]
Name=Startmydoc
Exec=emacs myfile.txt
Terminal=false
Type=Application
StartupNotify=false
A lot of related information is here:
Autostart Of Applications During Startup
Desktop Entry Specification