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.

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:

• Create a Project in the Google Cloud Console API Management Google Console
• Click on the Top bar Create Project (if you already have a project name there, click on it)
• On the popup click on (New Project)
• On the new page Set a Name for your project and select an organization (optional)
• On the top select + ENABLE APIS AND SERVICES
• On the list Search for Maps Javascript API and click on it
• On the top of the new page Click on ENABLE
• Go back to the Dashboard and from the left menu bar select CREDENTIALS
• Click on Create Credentials and select API, You will be given an API key to use for your site
• Click on RESTRICT KEY and setup the restrictions for your site or app.

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.