As of June 11, 2018, all Google Maps API requests must include an API key.
Keyless access is no longer supported.
Log into the Google account which will be responsible for the Google Maps API keys.
Once you're logged in, you'll be able to click on the following link...
Select which project you want these API keys to exist on.
If you don't have an existing project, create a new one now.
Click Library on the left nav. You'll see a giant list of all APIs that Google provides.
Search for and enable the following 3 APIs:
Click Credentials on the left nav.
Click the button that says "Create credentials", then select the "API key" option.
You can optionally skip steps 5 - 6, and leave your key unrestricted. This makes for a slightly faster and easier setup, but degrades the security of your API key.
If you choose to leave your key unrestricted, you will only need a single key. Jump to step 7, and enter your key into both fields.
When the modal appears, click "Restrict key".
Create two separate API keys:
For server-side calls to the API...
For browser-side calls to the API...
Return to step 4 to generate a second API key.
Copy your new API keys, and paste them into their respective fields.
Click the Save button, and you're all set!