To add a Spotify playlist to an event, you will need to go to the booking under Sales --> Bookings then select the booking you want to add the playlist to and click on the Questionnaires tab:
Add a new field as a Spotify/Apple Music/Youtube Playlist.
Click Set Playlist on the right, which will open a popup for you to paste your playlist url into.
After you enter your url and click Save, the playlist will be imported into your questionnaire.