I want to use Fiddler as a proxy for debugging purpose. Without fiddler everything works perfectly fine.

I have to add a certificate to my HTTP request. My code looks like this:

constructor TSpecialHTTP.Create(adbconn: IDBConnection; ...);
var
  ssl: TIdSSLIOHandlerSocketOpenSSL;
  dbresult: Variant;
  q: IDataset;
begin

  ... other unimportant database stuff

  // http is a private variable of the TSpecialHTTP class
  http := TIdHTTP.Create;
  // fiddler debug
  http.ProxyParams.ProxyServer := '127.0.0.1';
  http.ProxyParams.ProxyPort := 8888;

  ssl := TIdSSLIOHandlerSocketOpenSSL.Create(http);
  ssl.SSLOptions.SSLVersions := [sslvTLSv1_2];
  http.IOHandler := ssl;

  http.Request.ContentType := 'application/json; charset=utf-8';
  http.Request.UserAgent := 'MyApplication';
  http.Request.CustomHeaders.Clear;
  http.Request.CustomHeaders.FoldLines := false; // without it doesn't work, dunno why
  http.Request.CustomHeaders.Values[c_version] := GetMyVersion;
  http.Request.CustomHeaders.Values['Authorization'] := 'Bearer ' + token;
  CreateGUID(guid);
  http.Request.CustomHeaders.Values[c_message_id] := Copy(GUIDToString(guid), 2, 36);
  http.Request.CustomHeaders.Values[c_generating_utc] := DateToISO8601(TTimeZone.Local.ToUniversalTime(now), true);
  http.Request.CustomHeaders.Values[c_version] := cdhrecords[cdht].version;
  http.HTTPOptions := [hoForceEncodeParams, hoNoProtocolErrorException, hoWantProtocolErrorContent];

  certstream := TBytesStream.Create;
  if not VarIsNull(dbresult[1]) then begin
    SaveVariantToStream(certstream, dbresult[1]);
    certstream.Position := 0;
    certstream.SaveToFile(certfile);

    ssl.SSLOptions.CertFile := certfile;
    ssl.SSLOptions.KeyFile := certfile;
    ssl.SSLOptions.Mode := sslmBoth;
    ssl.OnGetPassword := GetPassword;
  end;
  
  ... again unimportant stuff
end;

I tried to add following line to Fiddler script on static function OnBeforeRequest(oSession: Session) {:

oSession["https-Client-Certificate"] = "C:\temp\cert.pfx";

This is what I get in Fiddler as a request:

POST https://api.example.com/cdh/v1.0/Message/Product HTTP/1.0
Content-Type: application/json; charset=utf-8
Content-Length: 56494190
version: 1.0
Ocp-Apim-Subscription-Key: <key>
message-id: 18EE53A3-64D0-4B34-9527-2D92F15FD462
generating-utc: 2024-09-04T07:46:44.363Z
signature: <signature>
Host: api.example.com
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: MyApplication

And this is what I get as a response:

HTTP/1.1 401 Missing certificate or authorization header
Keep-Alive: true
Content-Length: 0
Request-Context: appId=cid-v1:1c288406-24f1-4c26-9015-e7bdd006594a
Date: Wed, 04 Sep 2024 07:46:46 GMT
Connection: close

What do I have to do to make it work with Fiddler as a proxy?