Create a reminders list called "Spotify" in your iOS Reminders app. Then tell Siri, "Add Spotify reminder [Artist & Song]". This will create a new reminder in your Spotify list, which in turn will search Spotify and add to a playlist

This Trigger fires every time a new reminder is added to the list you specify.

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

Use Siri (Reminder) to search Spotify and add to Playlist