无法使用使用身份验证的C#代码执行POST请求

using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        string url = "https://example.com/api/endpoint";
        string username = "your_username";
        string password = "your_password";

        using (HttpClient client = new HttpClient())
        {
            // 添加身份验证信息
            client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes($"{username}:{password}")));

            // 构造POST请求的内容
            var content = new FormUrlEncodedContent(new[]
            {
                new KeyValuePair<string, string>("param1", "value1"),
                new KeyValuePair<string, string>("param2", "value2")
            });

            // 发送POST请求
            HttpResponseMessage response = await client.PostAsync(url, content);

            // 处理响应
            string responseContent = await response.Content.ReadAsStringAsync();
            Console.WriteLine(responseContent);
        }
    }
}