for me it seems like you want to set up the Google Assistant for The Turing Test. First of all you need to go to your Stadia app to the settings menu. Choose the Google Assistant entry in the settings menu and enable it.
Furthermore there are some requirements for the use of the Google Assistant on Stadia. You need a Stadia Controller because only the Stadia Controller has a Google Assistant button. Another requirement is the Google Chromecast Ultra because currently the Assistant is only available on the TV.
If you got these both you can access the Google Assistant while pressing the Assistant button on the controller. This also works when you are ingame playing The Turing Test.
For any further help please reply to this post.