Dưới đây là code mẫu tạo một class OpenAIChatClient

 public class OpenAIChatClient
    {
        private readonly HttpClient httpClient;
        private const string openAIEndpoint = "https://api.openai.com/v1/chat/completions";
        private const string apiKey = "api-Key";

        public OpenAIChatClient()
        {
            httpClient = new HttpClient();
            httpClient.DefaultRequestHeaders.Add("Authorization", $"Bearer {apiKey}");
        }

        public async Task<string> GetChatCompletion(string userInput)
        {
            var requestBody = new
            {
                model = "gpt-3.5-turbo",
                messages = new[]
                {
                new { role = "user", content = userInput }
            }
            };

            var jsonRequest = JsonSerializer.Serialize(requestBody);
            var httpContent = new StringContent(jsonRequest, Encoding.UTF8, "application/json");

            var request = new HttpRequestMessage
            {
                Method = HttpMethod.Post,
                RequestUri = new Uri(openAIEndpoint),
                Content = httpContent
            };

            var response = await httpClient.SendAsync(request);
            var jsonResponse = await response.Content.ReadAsStringAsync();

            // Xử lý kết quả ở đây
            // Ví dụ:
            // var result = JsonSerializer.Deserialize<dynamic>(jsonResponse);
            // var chatCompletion = result.choices[0].message.content.ToString();
            // return chatCompletion;

            return jsonResponse;
        }
    }

Chúc các bạn thành công.