I use Spotify in my car using Carplay all the time and it's very annoying to not have the ability to quickly save a song I like with a tap when I'm in my car. It'd be great if you guys added a button to the Now Playing screen (next to the shuffle/repeat buttons) that allowed us to Save (and prehaps even like/dislike songs). Apple Music already has this feature on Carplay and so does the Spotify app on Android Auto.
Especially since Discover Weekly and Release Radar are so popular, I imagine being able to save a song you've heard for the first time is a very oft-used action. The fact that we are unable to do so in Carplay is a huge missing feature.
UPDATE: Noticed today that certain playlists like Discover Weekly already has Like/Dislike buttons in the Carplay UI (see screenshot attached) that are accessible through a button in between the shuffle/repeat buttons that's labeled with three dots [...]. My request would simply be to (1) make the [...] button available for all tracks and (2) add a new option in that [...] menu to "Save to Library". (To avoid the buttons stacking up too high, the Like/Dislike buttons could be two halves of a split button on one row, with the Save button above or below it.)