The real work happens here: it looks like this library has figured out the secret model name that allows you to make API calls to the ChatGPT model in the same way you would usually make calls to the other GPT-3 models: https://github.com/waylaidwanderer/node-chatgpt-api/blob/mai...
That project's README (and the commit history to that README) tells the full story: sounds like people have been hanging out on Discord collaborating on reverse engineering ChatGPT: https://github.com/waylaidwanderer/node-chatgpt-api
One thing to note: you still have to provide your own OpenAI API key in order to call the model, so if/when OpenAI decide to shut this hole down it will likely be easy for them to restrict who can call the model and potentially take measures against accounts that have used it in this way.