Didn’t get this working, the friendly name wasn’t rewritten correctly to the entity-id as I use space in my names. And Spotcast was asking for the device_name. Ended up with this, which is even easier anyway:
Script:
# Spotify Start Playback
spotify_start:
alias: 'Spotify: afspelen'
sequence:
- service: spotcast.start
data:
device_name: "{{ states('input_select.speakers') }}"
force_playback: true
mode: single
icon: mdi:spotify
And automation:
# Muziek
- id: spotify_device_selected
alias: 'Spotify: Device Selected'
description: 'Starts music playing on the selected Chromecast Audio device'
trigger:
- platform: state
entity_id: input_select.speakers
condition: []
action:
- service: script.turn_on
entity_id: script.spotify_start
mode: single
Instead of having the script in the Lovelace GUI, I have playlists + radio available in a input_select, with a button to play the selected music to the selected speaker.