Sometimes when you start the xinetd service you might get an error about no service game/tcp etc if this is the case just open /etc/services and add your service name with the port you are running it on.
Here game is the name of the service and 1337 is the port on which it is running. Text after # is just a comment.
Other application with Systemd
This is just an example of flask application but in the similar manner you can run any other service as well. Ex: apache2