不支持的媒体类型415 c# (发送短信)

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

public class SmsSender
{
    private const string apiUrl = "https://sms.tencentcloudapi.com/";

    public async Task<string> SendSmsAsync(string phoneNumber, string templateId, string[] templateParams)
    {
        using (HttpClient client = new HttpClient())
        {
            client.BaseAddress = new Uri(apiUrl);

            // 构建请求内容
            var content = new FormUrlEncodedContent(new[]
            {
                new KeyValuePair<string, string>("PhoneNumber", phoneNumber),
                new KeyValuePair<string, string>("TemplateID", templateId),
                new KeyValuePair<string, string>("TemplateParamSet", string.Join(",", templateParams))
            });

            // 设置Content-Type头部
            content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");

            // 添加认证信息
            client.DefaultRequestHeaders.Add("Authorization", "Your-API-Key");

            // 发送请求
            var response = await client.PostAsync("", content);

            // 获取响应结果
            var result = await response.Content.ReadAsStringAsync();

            return result;
        }
    }
}