Adapt to this snippet: http://www.androidsnippets.com/executing-a-http-post-request-with-httpclient
var httpclient = new DefaultHttpClient();
var nameValuePairs = new ArrayList<INameValuePair>(2);
nameValuePairs.Add(new BasicNameValuePair("username", "admin"));
nameValuePairs.Add(new BasicNameValuePair("password", "***"));
var ent = new UrlEncodedFormEntity(nameValuePairs);
httppost.SetEntity(ent);
var response = httpclient.Execute(httppost);