TInternalHTTPClient = class(THttpClient)
end;
 
var
  httpClient: THttpClient;
  httpEngine: TWinHttpEngine;
  httpRequest: THttpRequest;
  httpResponse: THttpResponse;
  Store: HCERTSTORE;
  Cert: PCERT_CONTEXT;
begin
  httpClient := THttpClient.Create;
 
  // Open the ''Personal'' SSL certificate store for the local machine and locate the required client-side certificate
  Cert := nil;
  Store := CertOpenStore(CERT_STORE_PROV_SYSTEM, 0, 0, CERT_SYSTEM_STORE_LOCAL_MACHINE, PChar(''MY''));
  if (Store <> nil) then
    Cert := CertFindCertificateInStore(Store, X509_ASN_ENCODING, 0, CERT_FIND_SUBJECT_STR, PChar(''mycertsubject''), nil);
 
  // If a valid certificate was found then OK to create and send the HTTP request
  if (Cert <> nil) then
  begin
    // Setup HTTP request properties
    httpRequest := httpClient.CreateRequest;
    ...

    // Use ''BeforeWinHttpSendRequest'' event to set any HTTP request properties such as client-side SSL certificate
    httpEngine := TWinHttpEngine(TInternalHTTPClient(httpClient).Engine);
    httpEngine.BeforeWinHttpSendRequest :=
      procedure (Req: HINTERNET)
      begin
        WinHttpCheck(WinHttpSetOption(Req, WINHTTP_OPTION_CLIENT_CERT_CONTEXT, Cert, SizeOf(CERT_CONTEXT)));
      end;

    // Execute HTTP request
    httpResponse := httpClient.Send(httpRequest);
  end;