The GeoBooster website widget shows your Moments on a map, and that map is rendered by Google Maps. To use the widget you provide your own Google Maps API key, created in Google Cloud. If you do not embed the widget, you do not need this key. (It is separate from the Places API key used by the GeoGrid Widget - see Getting a Google Places API Key.)
Steps
Open the Google Cloud Console and sign in.
Create a new project, or select an existing one, and make sure billing is enabled on it.
Open APIs & Services > Library, then find and enable the Maps JavaScript API (the API that draws the map).
Open APIs & Services > Credentials, click Create credentials, and choose API key.
Copy the key that appears, and paste it into the GeoBooster widget.
Restrict the key to the APIs and domains you use, so it cannot be reused elsewhere and run up your Google bill.