touch $home/.xinitrc
after that (ensuring it exists, and if not, create it) open the file .xinitrc in your home directory with a text editor, like
vim $home/.xinitrc
and add there a line:
exec /path/to/your/app
that should do...the file .xinitrc should be run every time X starts, which means that when you start fluxbox (and X) it is run