Getting started with Mautic API

Trying a brute force approach; just putting this in the address bar:

https://mysite.com/mautic/api/contacts/?clientKey=actual_key&clientSecret=actual_secret&id=243&firstname=Changedname

This results in the following:

{“errors”:[{“message”:“API authorization denied.”,“code”:401,“type”:“access_denied”}]}

What is the minimal example to communicate with the API?

Where are the PHP API files supposed to be? I see I don’t have a folder /api/ in my Mautic installation; which files should I upload where? Or are they somewhere else under another name?

Should I include the API with a line like this - from dodgy years old online info…:

require_once __DIR__ . ‘/lib/Mautic/MauticApi.php’;