How to give referrer url for localhost in google map api?

I had that problem today, just typed "localhost" on the credentials part and it worked.

localhost:52200/* worked for me in July 2019. Didn't need explicit protocol like http or https.

Try to create an API key again. I would suggest you to follow Get a Key/Authentication.

NOTE:

Google Maps APIs are available for Android or iOS apps, Web browsers, and via HTTP web services. APIs in each platform require a specific type of key. The Google Maps JavaScript API will only work with a Browser key. APIs of the same platform can use the same key.

Also, check Registering authorized URLs to validate if you are entering a correct format URL.

For example:

http://localhost or http://localhost:[PORT_NUMBER]

Hope this helps!