Go to https://developers.google.com/maps/documentation/javascript
Then click on 
on the upper right.
Then select “Create a new project”
Then you obtain your API key:
Click on the link “API Console”:
In the new window, for the Key restriction, select “None”.
Save and you’re all set!
Now you just have to copy and paste your API key in your editor in Settings > Advanced, or in your backoffice > settings > APIs (in this case the API will be used by all apps which have no API key)