您可以通过执行添加用户代理标头
    client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("authentication.cs"));
您不能将 Content-Type 添加到默认请求标头,因为您只能在使用 PUT 或 POST 发送一些内容时设置 Content-Type。我猜你想像这样设置 Accept 标头:
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("text/html"));
更新:没有我自己的帐户,这是我能做到的。
公共密封部分类 MainPage : Page { private readonly HttpClient _httpClient = new HttpClient();
    public MainPage()
    {
        this.InitializeComponent();
        InitHttpClient();
    }
    private void InitHttpClient() {
        var username = "youremail@somewhere.com";
        var password = "yourharvestpassword";
        String authparam = System.Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(username + ":" + password));
        _httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", authparam);
        _httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
        _httpClient.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("MyHarvestClient", "1.0"));
    }
    /// <summary>
    /// Invoked when this page is about to be displayed in a Frame.
    /// </summary>
    /// <param name="e">Event data that describes how this page was reached.  The Parameter
    /// property is typically used to configure the page.</param>
    protected override void OnNavigatedTo(NavigationEventArgs e) {
        _httpClient.GetAsync("https://yoursubdomain.harvestapp.com/projects")
            .ContinueWith(t => HandleResponse(t.Result));
    }
    private void HandleResponse(HttpResponseMessage response) {
        response.EnsureSuccessStatusCode();
        var contentString = response.Content.ReadAsStringAsync().Result;  
        var contentXML = XDocument.Parse(contentString);
    }
}