Getting content/message from HttpResponseMessage

You need to call GetResponse().

Stream receiveStream = response.GetResponseStream ();
StreamReader readStream = new StreamReader (receiveStream, Encoding.UTF8);
txtBlock.Text = readStream.ReadToEnd();

I think the easiest approach is just to change the last line to

txtBlock.Text = await response.Content.ReadAsStringAsync(); //right!

This way you don't need to introduce any stream readers and you don't need any extension methods.