Hi @Okdone
you should be able to add this using the google home app on IOS or Android. Click add device and set the chrome cast up on your WiFi network. Once done it will ask if you would like to add stadia, again this can be done on the stadia app.
Hope this helps
Hello @Okdone
The second generation Chromecast is not supported by Stadia. Only the Chromecast Ultra (and newer "Chromecast with Google TV") devices support Stadia (from Google, that is – there are other devices from other manufacturers that also support Stadia).