如果您正在使用Visual Studio IISExpress调试模式并连接到HTTP端口而不是HTTPS端口，您可能会发现身份验证头被删除。

切换到SLL连接，它们将再次出现。

不确定原因，可能是设置重定向HTTP流量，导致身份验证被删除。

use UTF8选项

request.DefaultRequestHeaders.Authorization = 
new AuthenticationHeaderValue(
    "Basic", Convert.ToBase64String(
        System.Text.Encoding.UTF8.GetBytes(
           $"{yourusername}:{yourpwd}")));

使用现有的库可能更容易。

例如，Identity Server 4中添加了下面的扩展方法 https://www.nuget.org/packages/IdentityModel/

 public static void SetBasicAuthentication(this HttpClient client, string userName, string password);
    //
    // Summary:
    //     Sets a basic authentication header.
    //
    // Parameters:
    //   request:
    //     The HTTP request message.
    //
    //   userName:
    //     Name of the user.
    //
    //   password:
    //     The password.
    public static void SetBasicAuthentication(this HttpRequestMessage request, string userName, string password);
    //
    // Summary:
    //     Sets a basic authentication header for RFC6749 client authentication.
    //
    // Parameters:
    //   client:
    //     The client.
    //
    //   userName:
    //     Name of the user.
    //
    //   password:
    //     The password.
    public static void SetBasicAuthenticationOAuth(this HttpClient client, string userName, string password);
    //
    // Summary:
    //     Sets a basic authentication header for RFC6749 client authentication.
    //
    // Parameters:
    //   request:
    //     The HTTP request message.
    //
    //   userName:
    //     Name of the user.
    //
    //   password:
    //     The password.
    public static void SetBasicAuthenticationOAuth(this HttpRequestMessage request, string userName, string password);
    //
    // Summary:
    //     Sets an authorization header with a bearer token.
    //
    // Parameters:
    //   client:
    //     The client.
    //
    //   token:
    //     The token.
    public static void SetBearerToken(this HttpClient client, string token);
    //
    // Summary:
    //     Sets an authorization header with a bearer token.
    //
    // Parameters:
    //   request:
    //     The HTTP request message.
    //
    //   token:
    //     The token.
    public static void SetBearerToken(this HttpRequestMessage request, string token);
    //
    // Summary:
    //     Sets an authorization header with a given scheme and value.
    //
    // Parameters:
    //   client:
    //     The client.
    //
    //   scheme:
    //     The scheme.
    //
    //   token:
    //     The token.
    public static void SetToken(this HttpClient client, string scheme, string token);
    //
    // Summary:
    //     Sets an authorization header with a given scheme and value.
    //
    // Parameters:
    //   request:
    //     The HTTP request message.
    //
    //   scheme:
    //     The scheme.
    //
    //   token:
    //     The token.
    public static void SetToken(this HttpRequestMessage request, string scheme, string token);

request.DefaultRequestHeaders.Authorization = 
    new AuthenticationHeaderValue(
        "Basic", Convert.ToBase64String(
            System.Text.ASCIIEncoding.ASCII.GetBytes(
               $"{yourusername}:{yourpwd}")));

方法是这样的，

httpClient.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", "Your Oauth token");

你也可以使用下面的例子，它使用IHttpClientFactory:

    readonly IHttpClientFactory _httpClientFactory;
    
    public HTTPClientHelper(IHttpClientFactory httpClientFactory, string clientName = null)
    {
        this._httpClientFactory = httpClientFactory;
    }

    public Task<T> GetAsync(string url, string token) {

        var client = _httpClientFactory.CreateClient(_clientName);

        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(JwtBearerDefaults.AuthenticationScheme, token);

        using (HttpResponseMessage response = await _client.GetAsync(url)){
          ......
        }
     }