var postContent = new MultipartFormDataContent();
postContent.Headers.Add("ContentType", $"multipart/form-data");

//这是【string : string】的键值对
postContent.Add(new StringContent(appConfig.apiName), "api_name");

//这是【string : Json】的键值对；param是一个类的对象
postContent.Add(new StringContent(param.ToJsonStr()), "content");

//form表单格式传参

string result = "";

string url = "http://xxxxx.cn";
try
{
　　HttpClient client = new HttpClient();
　　HttpResponseMessage response = client.PostAsync(url, postData).Result;
　　result = response.Content.ReadAsStringAsync().Result;
}
catch (Exception ex)
{
　　result = ex.Message;
}