According to Google guidelines - "If you already own a domain name (such as example.com), and you want to use this domain name as the name of your Cloud Endpoints service, you must prove that you are authorized to use the domain."
Login here to get these keys: https://console.cloud.google.com/apis
For additional support regarding these verification keys and setting this up in your console contact Google Support: Maps API Google Support
We recommend using the Meta Tag option -
To add the Meta Tag go to:
Settings » Design Settings » Custom CSS/HEAD » Add it inside the Additional Head Code