API authentication is handled by sending your private api key in the http header on the request. The key is sent as a bearer token in the authorization http header.
If your plan includes API access, you can find your private api key by navigating to yoururl.com/admin/site_api . And then go to Site API -> Settings from within your admin panel. Please note, you should keep this API key secret as it is essentially a password allowing full access to the data in the platform via the API.
Curl Example to specify HTTP authorization header
curl \ -X $HTTP_METHOD \ -H "Authorization: Bearer $PRIVAPIKEY" \ "https://www.sitedomain.com/site_api/$ENDPOINT"