Вы можете установить значения при создании нового объекта FacebookClient
/// <summary>
/// Initializes a new instance of the <see cref="FacebookClient"/> class.
/// </summary>
/// <param name="appId">
/// The app id.
/// </param>
/// <param name="appSecret">
/// The app secret.
/// </param>
public FacebookClient(string appId, string appSecret) : this(string.Concat(appId, "|", appSecret))