To turn on JSON mode with the Chat Completions or Assistants API you can set the response_format to { "type": "json_object" }.

openai-api-json-format.cs

        Dictionary < string, string > responseFormat = new Dictionary < string, string > {
          {
            "type",
            "json_object"
          }
        };
        var payload = new {
          model = "gpt-4o-mini",
            messages = messages,
            max_tokens = 300,
            response_format = responseFormat
        };