Make a button on dashboard that turns my tv off. I can by voice "hey Google turn off chrome cast". Tv and Chromecast go off! Why: to be able to put the remote in the drawer.
So this is where someone says oh ya you just do this in this menu. .
Actually I have 2 nvidia shields and just telling google to turn them on or off by name they do it, I did not program anything. Shield works as a Chromecast.
For the button in the dashboard, IFTTT probably can do it.
You could do this with assistant relay. Using the driver ported by @ogiewon you can send a command with the prefix [CC]. However, you might have to send the command as "turn off the {ROOM} Chromecast." The assistant relay device broadcasts to your entire GH infrastructure.