Google requires that you do all requests from your site or app using an individual API key where it tracks the requests and provides free requests per month.
Note: Please do NOT use the same Google API project for all of your sites in order to avoid hitting the API request limit.
Updates in the Key or key generation will require up to 48 hours to propagate to your site.
Google provides a wizard to create the API keys for your site or application. Use the following link: Google Maps Platform
After you can always visit Google Console to retrieve your API Keys or add additional keys in the future.
Using the following steps you can create an API key:
It is important to restrict your key for your specific domain/app, the key is visible on your website and others can copy and use it thus using your free requests per month. Usually these users of your key use it for nefarious reasons and they have been banned by Google which could mean you will be banned to in the future.