Method for get base URI
Maybe you can do it like this:

private const string OAuthTokenUri = "/oauth2/token";

private static string GetOAuthTokenUri(string uriAddress)
{
   Uri result = null;
   return Uri.TryCreate(new Uri(uriAddress), OAuthTokenUri, out result) ? result.ToString() : string.Empty;
}


Please note the slash I have added to OAuthTokenUri.