Here is the setting up an internet radio station FAQ.

Which sends you here

https://help.tunein.com/en/support/solutions/articles/151000172376-what-is-a-custom-url-and-how-can-i-add-it-to-sonos-

And that’s outdated and no longer works.

Odd. Since it’s TuneIn’s web page that is outdated, and doesn’t work, have you contacted them? 

​@Muzzer100 I seem to be able to reach the page in the app that should be used the ad url’s. I’m not in the position to ad a url at the moment, so can you tell us why the page that is referring to this feature is outdated?

You can add urls using the Sonos Desktop apps.

You can play those urls using the Sonos S1 apps, as they still support “classic TuneIn”.

You can do all of these things with my latest apps (see profile) on S1 and S2.