fbautostart software package provides XDG compliant autostarting app for Fluxbox, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install fbautostart
fbautostart is installed in your system.
Make ensure the fbautostart package were installed using the commands given below,
$ sudo dpkg-query -l | grep fbautostart *
You will get with fbautostart package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Miscellaneous |
| Package | fbautostart |
| Version | 2.718281828-1+b1 [amd64, arm64 |
| Description | XDG compliant autostarting app for Fluxbox |
| Sub Section | Miscellaneous |