/browser.cpp
BrowserWindow *Browser::createWindow(bool offTheRecord)
{
    ...
    auto certStore = profile->clientCertificateStore();
    QFile keyFile("user.p12");
    if (keyFile.open(QIODevice::ReadOnly))
    {
        QSslKey client_key;
        QSslCertificate client_cert;
        if (QSslCertificate::importPkcs12(&keyFile, &client_key, &client_cert))
        {
            certStore->add(client_cert, client_key);
        }
    }
   ...
}

// webpage.cpp
void WebPage::handleSelectClientCertificate(QWebEngineClientCertificateSelection selection)
{
    for (auto& cert : selection.certificates())
    {
      ...
    }   
}