To authenticate you'll need to log in and get your API key. Include your key in an Authorization header with every request you make: Authorization: Bearer YOUR_API_KEY.
For example (replace the fake API key):
$ curl \
-H "Authorization: Bearer dac610f0-1dd7-412b-b157-3fa45e82f7a3" \
-d '{"length": 100, "prompt": {"text": "Hello world!"}}' \
https://api.inferkit.com/v1/models/standard/generate
A response with error code UNAUTHENTICATED means that your Authorization header is either missing or incorrect.
Your API key is secret. Do not share it with anyone you don't trust or embed it in a web page.