How to cast music to my google cast (speakers) using spotify

I have google cast enabled speakers. I’ve also downloaded the group app to be able to group some speakers. But how can I now start casting music to these (grouped) speakers using for example spotify? How to create that flow?

Homey:

Add an (advanced) virtual device, use device class kettle

Make sure the Google assistant integration is still enabled (it tends to disable itself after x days).
To make sure it gets synchronised, yell “hey doodle, synchronise devices”.

Google Home:

Add an automation, and have it triggered by that (advanced) virtual device.
Action: Play media
Select “Music”

In the text field that shows, now just enter a Spotify group, or a playlist, or whatever works @ Spotify

Next,
at “Configuration”, select the speaker group you created in advance.

Example automation:
(my (advanced) virtual device is called “Spotify favs via Ggl Home”)