Use Pixel 2's Now Playing feature to add music to a Spotify Playlist -

Connect

Add to a playlist using the Google Pixel 2's 'Now Playing' ambient feature. When your Pixel 2 device detects a song is playing, that song (if found on the Now Playing database) will be added to a Spotify Playlist.

If

Notification received from a specific app

This trigger fires every time a new notification is received on your Android device from an app that you specify. NOTE: will not fire for IFTTT app notifications.

When

Filter code

Then

Add track to a playlist

This Action will search for a track and add the first result to a playlist you specify.

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Use Pixel 2's Now Playing feature to add music to a Spotify Playlist