Create an application in FMS
At the most basic level, in FMS, an "application" is a directory. You can add functionality (and complexity) in the form of server-side code.
1. As root, create a directory with this command:
mkdir /opt/macromedia/fms/applications/foobar
2. Change permissions on the directory to allow FMS to write files to it:
chmod -R 775 /opt/macromedia/fms/applicationms/foobar
3. That's it! To use it, point your FMS application to the application thusly:
rtmp://www.yourservername.com/foobar