Can't play music from Plex Media Server onto Chromecast audio

Configuration
1

I’m using the following automation. It doesn’t output any sound. I know the service is correct, as is
the entity_id, as I got them from the HAOS web GUI. I’m less sure about the “data” part. How can I ascertain I’m using the correct media_content_id, in particular ?

service: media_player.media_play
data:
  media_content_type: music
  media_content_id: >-
    plex://{"library_name": "Music", "artist_name": "Gustav Leonhardt",
    "album_name": "Gustav Leonhardt Edition (2008)", "track_name": "shuffle"}
target:
  entity_id: media_player.office_speakers

Note that if I go to the Media tab in the HA UI on the left, and navigate within my Plex server , I can play onto my Chromecast audio just fine manually. The issue is doing it through an automation.

2

I found that the following works . Apparently escaping is important .

service: media_player.play_media
data:
  media_content_type: music
  media_content_id: >-
    plex://{"library_name": "Music", "artist_name": "Johann Sebastian Bach",
    "shuffle": "1", "maxresults": 1}
target:
  entity_id: media_player.office_speakers