Run 1 of my shortcuts from HomePod Mini via Siri

I actually don't subscribe to Apple Music, so I'm constrained... but, I can get my homepod to play any radio station by name using Apple Music (via Siri) if I specify the station's label from TuneIn. In other words, Apple Music doesn't let you browse for every radio station available via TuneIn, but it'll play it if you specify the station by name.

Search for your station on https://tunein.com/search/ and give it a try.

I don't know if you can then specify station that in a Home Scene, but perhaps obscure stations recently played on your iPhone/iPad will show up in the Home App's 'specify audio' field.

Via Apple's product feedback page, (Feedback - Home App - Apple), I once asked for the ability to create Home Scenes that simply execute a command as given to Siri (basically have Siri execute a typed command stored in the Scene). Still waiting :-).

I suspect the HomePod lacks the browser engine to open a URL. I believe the closest you can get might be a Home Scene that plays a specific 'radio station' via Apple Music/TuneIn. Unfortunately, Apple doesn't make the "Select Audio" attribute available to people who don't subscribe to Apple Music.

The other approach would be to modify the shortcut on the iPhone to open the URL then play it on the HomePod via AirPlay (Change Playback Destination).