In the Spotify desktop UI: create a new playlist (or use an existing playlist.) Copy the Spotify URI for the playlist (e.g. spotify:playlist:5x5mdsVit4ngNyvglqkO8f).
Run the app by giving the playlist id (without the spotify:playlist: part) as command line argument:
go run . 5x5mdsVit4ngNyvglqkO8f christmas
The app will open browser for Spotify authentication. Log in and give app permission to modify your playlists.
Bunch of random Christmas songs are added to your chosen playlist:
go run . 5x5mdsVit4ngNyvglqkO8f
2020/11/27 21:55:53 http: Server closed
...
Add track Oscar Peterson: God Rest Ye Merry, Gentlemen
Add track Sister Rosetta Tharpe: O Little Town Of Bethlehem
Add track Dianne Reeves: Jingle Bells
Sleeping a while...
...
All done! Merry Christmas! Hyvää joulua!
https://open.spotify.com/playlist/5x5mdsVit4ngNyvglqkO8f
Start listening and find new favourite Christmas tunes 🎄!