Please tell me how to use Geocoding API

I'm going to get the road name address and save it as latitude and longitude.

In order to do this, I have to use geocoding, but the usage method changed last year, so I'm leaving a question here because I don't have enough information on how to use it.

Currently, the API key has been issued on the Google platform, but if you put the key in the link,

{ "error_message" : "This IP, site or mobile application is not authorized to use this API key. Request received from IP address ..*.*, with empty referer", "results" : [], "status" : "REQUEST_DENIED" }

This error will appear.

I'm leaving a message because I can't understand even if I refer to the manual.

https://developers.google.com/maps/documentation/geocoding/start?hl=ko