There are 2 different methods of providing credentials to the Google Maps API when working with TMS WebGMaps:

1. Regular Google Maps access API Key
https://console.developers.google.com
This key identifies your project to check quotas and access.
At the Google Developers Console, go to "API Manager", "Credentials" to view your existing API Key(s) or create a new one.
You can assign this key to the APIKey property.


2. Google Maps for Work access Client ID and Signature
https://www.google.com/intx/en_uk/work/mapsearth/
https://developers.google.com/maps/premium/overview
These credentials identify your Google Maps for Work project.
After sign up for Google Maps for Work you should receive the required information.
You can assign these to the APIClientID and APISignature properties.