Maybe the easiest way to prioritize a certain application is to use the programs / prioritization dialog. If this isn't applicable you can create your own filter rule as follows:
First you have to determine which port your application uses and if it's TCP or UDP. Use
or the cFos / cFosSpeed connection overview dialog to find your application. Then you set up the filter as follows:
filter=-tx -udp-sport / -udp-dport / -tcp-sport / -tcp-dport X -c classname.
You have to decide if it's a source port or a destination port. In most cases there is a server on the Internet with a certain port, so it's a destination port (dport). The filter rule would look like this:
filter=-tx -udp-dport X -c classname for UDP or filter=-tx -tcp-dport X -c classname for TCP
where X is the server's port and classname the class you want the traffic sorted into, e.g. high or higher (but never highest) if you want to increase the priority or lower / lowest if you want to decrease the priority.
Now insert this new filter rule into settings.ini, section [filters]. For UDP in most cases it should be at the top of the list of the other filter rules. For TCP it should rather be at the end. You can use "spd fstat" to see if your rule matched.
After changing settings.ini you need a "spd reload" command to let cFos / cFosSpeed reload the filter rules.