http://www.subsonic.org/pages/getting-started.jspWhile you are looking that page over, and branching out from there, I would caution you that Subsonic is abandonware. There is no development or support from the owner, and as it is closed source, there is never going to be any development unless the owner returns. If you start to feel that the feature set in the Premium subscription is something you would want, I recommend looking at other solutions rather than trying to purchase the subscription or using the Google-able hack.
To the former, be aware that there was a time that Subsonic was open source, and there are current actively developed branches out there. The Subsonic API is also well published and thus there are other solutions that use the API which can be leveraged. So, I'll offer an FYI you might consider:
If you're looking for a very lateral move, consider Airsonic Advanced
https://github.com/airsonic-advanced/airsonic-advanced - It is in active development with frequent snapshot updates
https://github.com/airsonic-advanced/airsonic-advanced/releases . Same feature set as Subsonic (API, Sonos, etc.) with updated code. As it is open source, you also get almost all the features Subsonic Premium gives you, but for free (see my last line). Minimal effort for installation (latest Java installed, then a command line shortcut to the war file - upgrades even easier with just a fast war file change).
If you're really more API focused, and looking just for a music streaming service (app over browser), you might consider moving off the Subsonic family of servers altogether. Check out Navidrome
https://www.navidrome.org/ . That product is primarily to supply the API with a completely new back-end, and refocuses to just support audio (no video, podcast, internet radio, etc.). There is not yet built in support for Sonos, but you can find easy linking with something like Bonob
https://github.com/simojenki/bonob. As with Airsonic Advanced, no subscription or fee to access the API. Navidrome does have a simplified web UI if desired.
Both products will need you to come up with your own DDNS solution for external access. Both support running as a service with something like NSSM
https://nssm.cc/, and IIS works great as a reverse proxy if you wish to run them as SSL.