https://developer.apple.com/documentation/applemusicapi/add_tracks_to_a_library_playlist
I use this api with python, post data:
https://api.music.apple.com/v1/me/library/playlists/{id}/tracks
{'tracks': {'data': [{'id': '310730206', 'type': 'songs'}, {'id': '310730208', 'type': 'songs'}]}}
error: 'Unable to parse request body'