How to add a key during map initialization?

Question

JavaScript

How to add a key during map initialization?
i.e. initially the api map file was loaded on the map

<script src="https://maps.googleapis.com/maps/api/js" async defer></script>

By clicking on the button, I initialize the map, but for some reason, the key is not added in this way:
GoogleMapsLoader.KEY = 'Key';
What can be done about it?