How to connect google map api?

Question

JavaScript

I'm making a trip cost calculator with google maps, everything works, but you need to register a google api. So I can't figure out where to put it.
The calculator itself is here it-service21.ru/map/index.php?map=1 I
used to do it on Yandex, it was more understandable there, but due to restrictions I had to go to the google map.