The OpenAI API uses API keys for authentication. Visit your API Keys page to retrieve the API key you’ll use in your requests.
Maybe you are using a custom endpoint which is providing an OpenAI-API compatible API. For using a custom endpoint, you just need to provide the base url in the preferences. For example, if you are using FastChat, you need to put http://localhost:8000/v1 in Preferences > OpenAI Custom Model > API URL.
If you want to use a model of an OpenAI-API compatible API or from the OpenAI API which isn’t available in Bavarder (available: gpt-3.5-turbo, gpt-4, text-davinci-003), you can use the provider callled OpenAI Custom Model and set Model to the model you want to use. For example, if you want to use gpt-4-32k you just have to set Model to gpt-4-32k
See this page of the documentation.