CallbackUrl is empty

[Nininea-zz]

I use GetRequestToken with callback url parameter, but it return empty.

IOAuthSession session = new XeroApiPublicSession(UserAgent, ConsumerKey, ConsumerSecret, new InMemoryTokenRepository());
session.MessageLogger = new DebugMessageLogger();
var url = new UriBuilder("http://localhost:49382//oob") { }.Uri;
IToken requestToken=session.GetRequestToken(url)

this is my code. how to fix this problem?