Tap on the three dots to the right of the song title. A new window will open. Swipe up where it says “Spotify” and the second option will be “Add Song to Spotify Playlist”.

tburns,
Note, I think this is also only available if using Spotify premium - I think the Spotify free version will not let you select individual songs from a radio listing to add to a playlist.

Thanks for replies. I have Spotify premium. The only options I get when I tap on the 3 dots are Crossfade and Sleep timer.

Swipe up on the word “Spotify” (directly above Crossfade) and the second option will be “Add Song to Spotify Playlist”.

A screenshot here too, to maybe assist.