How to get structured JSON output from OpenAI in n8n with custom extracted fields?

I'm trying to use the OpenAI integration in n8n to extract specific information from a client's message and return it in a structured JSON format.

For example, I want the assistant to extract values like the client's name and city and return a response like this:

{
  "output": "Hi! How would you like to be called? 😊",
  "custom_fields_values": [
    {
      "field_name": "City",
      "values": [{ "value": "Curitiba" }]
    },
    {
      "field_name": "Name",
      "values": [{ "value": "Ana" }]
    }
  ]
}

What should I include in the prompt, and how should I configure the OpenAI node in n8n to make sure the response always follows this JSON structure?

Any tips for making the output reliable and machine-readable?