I see now as stated on the readme,
"It's also possible to use a .conf file to set options for mavlink-routerd. By default, mavlink-routerd looks for a file /etc/mavlink-router/main.conf. File location can be overriden via MAVLINK_ROUTER_CONF_FILE environment variable, or via -c switch when running mavlink-routerd. An example of conf file can be found on examples/config.sample"
Thank you for bringing my attention to this.
Have a nice day!