How to call Json Rest API in C# through API KEY with POst method?

string strUrl  =   // pass the path of api
WebRequest requestObjPost  = WebRequest.Create(strUrl);
requestObjPost.Method = "POST";
requestObjPost.ContentType =  "application/json";
string postdata  =. "{\"title\": \"data\",}".  // whatever data you want to post  pass the entities
using (var streamWriter   = new StreamWriter(requestObjPost.GetRequestStream()))
{
    streamWriter.Write(postdata); 
    streamWriter.Flush();
    streamWriter.Close(); 
    var httpResponse =  (HttpWebResponse)requestObjPost.GetResponse();
    using (var streamReader  = new StreamReader(httpResponse.GetResponseStream()))
    {
        var result2  = streamReader.ReadToEnd();
    }
}