Your Google Map API Key can be used on multiple authorized websites.
When you first created your Google Map Key, one of the most important steps was to protect your budget and quota by restricting it's use to only your domains so that it could not be used by others.
If you don't restrict the key by website, then others can use your Google API key and quickly deplete your budget, or incur costs on your Google account!
If you have changed your domain name, or you want Google Maps to work on another domain, (for example, a temporary domain for development), then you will need to adjust 'Website Restrictions' in your Google API account to add additional authorized domains that can use your Google Map API key.
1. Login To Your Google Account
The very first thing to do is to log in to your Google account (e.g. GMail etc.).
If you have multiple Google accounts, make sure to go to the one you used for your Maps API Key.
2. Navigate to Google Maps Platform
Then go to the Google Maps Platform page:
https://console.cloud.google.com/google/maps-apis/home.
Make sure that you have the correct project selected in the top blue bar.

On the left side menu, you should be seeing the "Google Maps Platform" menu.
(If not then click the white menu icon in the top blue bar - and scroll all the way down the menu (it's a long way down!) and select "Google Maps Platform").
3. View Website Restrictions
In the left menu, select the menu item for Credentials.

In the main panel, you will now see a table headed "API Keys".
Click the name of the API key that is shown (probably called "API Key" or similar).
You will then see the 'Key Restrictions', page - as shown below.
(If you don't see this then you are probably in the wrong Google account).

4. Authorize 2 Versions of The New Domain
Under the list of previously added websites, click ADD AN ITEM to add your new website address EXACTLY as described below to add TWO versions of your website domain name.
Remember - this method will enable the maps key on subdomains and any pages - so do not be tempted to add something different!
Assuming your domain is https://www.bobsrealty.com,
.. then in your website in the following format:
*.bobsrealty.com/*
i.e. do not enter 'http', or 'www' etc. - JUST the domain name prefaced with the characters ASTERISK DOT, and followed by characters FORWARD-SLASH ASTERISK.
Click DONE.
Then - repeat this to add your domain name again, but this time with nothing at the beginning of the domain name, like this:
bobsrealty.com/*
i.e. this time JUST the domain name (no preface), followed by the characters FORWARD-SLASH ASTERISK.
(do not enter 'http://', or 'www' etc. )
Click DONE.
At this stage you should now see two EXTRA entries for this website specified, similar to this:
*.bobsrealty.com/*
bobsrealty.com/*

Do not be tempted to change or modify any other settings or restrictions on this page!
Finally, at the bottom of this page select SAVE.
You are done! When the maps load using your API key on this new website, the domain is now authorized.
Useful direct links for Google Maps Platform
- Direct Link to the Google Maps Platform - https://console.cloud.google.com/google/maps-apis
- Link to set budgets - https://console.cloud.google.com/billing/budgets
(only important if you have very high use and need to manage costs)