A Microsoft open-source framework for building native device applications spanning mobile, tablet, and desktop.
You're trying to use the sync version of the method. Sync methods, in general, are not recommended. For Mac you must use the async version (SendAsync).
//HttpResponseMessage httpResponse = _client.Send(request, HttpCompletionOption.ResponseHeadersRead);
HttpResponseMessage httpResponse = await _client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false);
You can read more here about why this decision was made but honestly nobody uses Send anyway and you're in the middle of an async all so there is no reason to use the sync version.