However there are already parameters to force specific ports (--webservice-port=####) and have the tray icon NOT start it’s own server and instead try to connect to an existing service / daemon server (--no-hosted-server).
If you’re asking how to have the Duplicati service run even if nobody has logged into the machine (which in this case is a server) then you might want to read through this post to see if it covers what you need: