How to transfer audio to a Chromecast Audio... via Python!

[daghemo]

Plan

Free/Premium

Country

Italy

Device

OnePlus 6T, Raspberry Pi 3 B+

Operating System

Android 10+ on the OnePlus 6T, Raspbian Buster on the Raspberry Pi

My Question or Issue

I'm playing with Python on my Raspberry Pi and I'm now able to turn on my Onkyo home theatre system via a BroadLink IR blaster an switch the input channel to the Chromecast Audio connected via Toslink.

Now I would like Spotify to play music on it.
The problem is that the device does not show up on https://developer.spotify.com/console/get-users-available-devices/.

I can see my Alexa device and my smartphone. But not my Chromecast device.

If I go to the smartphone I see the Chromecast device and I can "move" my audio to that.
Also, it seems that I cannot ask Google Home to do that for me, as I saw on

https://community.spotify.com/t5/Live-Ideas/Google-Home-Play-on-specific-Device/idi-p/3603982.