How to covert HttpWebRequest implementation to HttpClient

Question

I have one service method which is using HttpWebRequest for below stuff

 

while (ub < sentCount)
{
    ub = step * (1 + (i++));

    var k = (ub > sentCount) ? (sentCount) : ub; //to avoid array out of range exception(assign unitll array length if calc exceeds)

    for (int j = lb; j < k; j++)
    {
        pnos = pnos + "," + pnosList[j].Phone;
    }
    pnos = pnos.Substring(1);

    var sbPostData = new StringBuilder();
    sbPostData.AppendFormat("authkey={0}", api.AuthenticationKey);
    sbPostData.AppendFormat("&mobiles={0}", pnos);
    sbPostData.AppendFormat("&message={0}", message);
    sbPostData.AppendFormat("&sender={0}", api.SenderId);
    sbPostData.AppendFormat("&route={0}", "default");
    string sendSMSUri = api.EndPoint;


    // Create HTTPWebrequest
    var httpWReq = (HttpWebRequest)WebRequest.Create(sendSMSUri);
    //Prepare and Add URL Encoded data
    var encoding = new UTF8Encoding();
    byte[] data = encoding.GetBytes(sbPostData.ToString());
    //Specify post method
    httpWReq.Method = "POST";
    httpWReq.ContentType = "application/x-www-form-urlencoded";
    //httpWReq.ContentLength = data.Length;
    using (Stream stream = httpWReq.GetRequestStream())
    {
        stream.Write(data, 0, data.Length);
    }
    //Get the response
    var response = (HttpWebResponse)httpWReq.GetResponse();
    var reader = new StreamReader(response.GetResponseStream());
    string responseString = reader.ReadToEnd();

    //Close the response
    reader.Close();
    response.Close();

    lb = ub;
    pnos = string.Empty;
}

Now the same thing I need to do in HttpClient is it possible to do that. Issue I am facing is HttpWebRequest is not supporting in PCL(C# class library) I want to move above logic to PCL.