Please read again.
app-file is a
file, not a directory, as the name indicates; if it doesn't
exist, create it appropriately in the fluxbox configuration
directory (usually
$HOME/.fluxbox). Otherwise, I already
gave you
a
precice reference on the various methods you can use.
Agreed, putting the line I gave above into
$HOME/.xinitrc make no sense; see
man startx.