I think you need to register your application in the dashboard here .
After registration, you will be assigned the application identifier for your application, which can then be used for development.
Excerpt from the documentation :
Before you can develop and test applications on your own Google Cast device, you need to get the application ID for the API calls and register your Google Cast Device (e.g. Chromecast) as a development device. For more details, read the Registration Guide.
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