How Do You Use Basic Authentication with System. Net. Http. Httpclient?

Don't encode the whole authentication string - encode the "Username:Password" expression and append the result to the "Basic " prefix.

var authenticationString = $"{clientId}:{clientSecret}";
var base64EncodedAuthenticationString = Convert.ToBase64String(System.Text.ASCIIEncoding.UTF8.GetBytes(authenticationString));
content.Headers.Add("Authorization", "Basic " + base64EncodedAuthenticationString);

Also, consider using just ASCII encoding - the UTF8 may not be understood by the server unless you add a charset declaration to the header.

Wikipedia seems to cover this quite well.

It's not a good practice to create HttpClients explicitly from your calling code. Please use HttpClientFactory that simplifies a lot of things.

However, if you want to use basic authentication, just create an HttpRequestMessage and add the following header:

var request = new HttpRequestMessage(HttpMethod.Post, getPath)
{
    Content = new FormUrlEncodedContent(values)
};
request.Headers.Authorization = new BasicAuthenticationHeaderValue("username", "password");
// other settings

If you decide to use a recommended IHttpClientFactory it's even simpler:

serviceCollection.AddHttpClient(c =>
{
   c.BaseAddress = new Uri("your base url");
   c.SetBasicAuthentication("username", "password");
})

The specific problem is this line (below)

content.Headers.Add("Authorization", $"Basic {base64EncodedAuthenticationString}");

This fails because HttpContent.Headers (System.Net.Http.Headers.HttpContentHeaders) is only for headers that are content-specific, such as Content-Type, Content-Length, and so on.

You've stated that you can't use DefaultRequestHeaders because you only need it for a single request - but you also can't use it with PostAsync - only SendAsync provided you construct the HttpRequestMessage yourself, as per your own answer and @NeilMoss' answer - but you could use an extension-method in future.

But for the benefit of other readers, another alternative is to add a new extension method based on the existing PostAsync, which is actually really simple (only 3 lines!):

public Task<HttpResponseMessage> PostAsync( this HttpClient httpClient, Uri requestUri, HttpContent content, String basicUserName, String basicPassword, String? challengeCharSet = null, CancellationToken cancellationToken = default )
{
    if( basicUserName.IndexOf(':') > -1 ) throw new ArgumentException( message: "RFC 7617 states that usernames cannot contain colons.", paramName: nameof(basicUserName) );

    HttpRequestMessage httpRequestMessage = new HttpRequestMessage( HttpMethod.Post, requestUri );
    httpRequestMessage.Content = content;

    //

    Encoding encoding = Encoding.ASCII;
    if( challengeCharSet != null )
    {
        try
        {
            encoding = Encoding.GetEncoding( challengeCharSet );
        }
        catch
        {
            encoding = Encoding.ASCII;
        }
    }

    httpRequestMessage.Headers.Authorization = new AuthenticationHeaderValue(
        scheme   : "Basic",
        parameter: Convert.ToBase64String( encoding.GetBytes( userName + ":" + password ) )
    );

    return SendAsync( httpRequestMessage, cancellationToken );
}

Usage:

HttpClient httpClient = ...

using( HttpResponseMessage response = await httpClient.PostAsync( uri, content, basicUserName: "AzureDiamond", basicPassword: "hunter2" ).ConfigureAwait(false) )
{
    // ...
}

Just something to add that I struggled with, which I only experienced with Basic authentication endpoints. If you add Json as StringContent then it adds a charset=utf-8, this often return a BadRequest 400.

Here is the code I got to fix this: reference:

using (var client = new HttpClient())
using (var content = new StringContent(ParseJSON(data), Encoding.Default, "application/json"))
{  
   //Remove UTF-8 Charset causing BadRequest 400
   content.Headers.ContentType.CharSet = "";

   var clientId = "client";
   var clientSecret = "secret";
   var authenticationString = $"{clientId}:{clientSecret}";
   var base64EncodedAuthenticationString = Convert.ToBase64String(System.Text.ASCIIEncoding.UTF8.GetBytes(authenticationString));
   client.DefaultRequestHeaders.TryAddWithoutValidation(authHeader, authorization);

   var response = await client.PostAsync(url, content);
   return response;
}

I have resolve this by using below code, that serve my purpose also. Added Code for both Get/Post, this will help you. Moreover I have added one more Header key. So to pass extra data to header. Hope that will resolve your issue.

          class Program {
                  private static readonly string Username = "test";
                  private static readonly string Password = "test@123";
                  static void Main(string[] args) {
                     var response = Login();
                   }
            public static async Task Login() 
             {
                 var anotherKey ="test";
                  HttpClient httpClient = new HttpClient
                {
                    BaseAddress = new Uri("")
                };
    
                httpClient.DefaultRequestHeaders.Add($"Authorization", $"Basic {Base64Encode($"{Username}:{Password}")}");
                httpClient.DefaultRequestHeaders.Add($"anotherKey", $"{anotherKey}");
                HttpResponseMessage httpResponseMessage = await httpClient.GetAsync("user/123").ConfigureAwait(false);
// For Get Method
            var response= await httpResponseMessage.Content.ReadAsStringAsync().ConfigureAwait(false);
// For Post Method
 User user = new User (1,"ABC");
            HttpResponseMessage httpResponseMessage = await httpClient.PostAsJsonAsync("/post", user).ConfigureAwait(false);
            UserDetail userDetail  = await httpResponseMessage.Content.ReadAsAsync<UserDetail>().ConfigureAwait(false);
        }
            
  }