How to activate/force Basic Auth on THTTPClient - delphi

Question

I can't find an option to activate Basic Auth on THTTPClient.
My main "issue" is that the first request response with a HTTP/1.1 401 Unauthorized and it triggers an auth event. I want to do it on the first "try".
Here is my code:
uses
System.Net.HTTPClient, System.Net.URLClient, System.NetEncoding;
procedure DoRequest(const url, username, password, filename: string);
var
http: THTTPClient;
ss: TStringStream;
base64: TBase64Encoding;
begin
http := THTTPClient.Create;
ss := TStringStream.Create('', TEncoding.UTF8);
base64 := TBase64Encoding.Create(0);
try
// option #1 - build in credential storage
http.CredentialsStorage.AddCredential(
TCredentialsStorage.TCredential.Create(
TAuthTargetType.Server, '', url, username, password
));
// option #2 - custom "Authorization"
http.CustomHeaders['Authorization'] := 'Basic ' + base64.Encode(username + ':' + password);
http.ProxySettings.Create('127.0.0.1', 8888); // for fiddler
http.Get(url, ss);
ss.SaveToFile(filename);
finally
base64.Free;
ss.Free;
http.Free;
end;
end;
Option #2 includes the Authorization in the first request. Option #1 does not.
How can I do that?
Update - PreemptiveAuthentication does not exist in Seattle

Answer 1

In Delphi 11 and later, using THTTPClient.CredentialsStorage should work, you just need to set THTTPClient.PreemptiveAuthentication to true:
Property controls preemptive authentication. When set to True, then basic authentication will be provided before the server gives an unauthorized response.
However, PreemptiveAuthentication does not exist in Delphi 10.x, including Seattle, so you will have to stick with your CustomHeaders solution until you can upgrade to a modern Delphi version.