How to set size unit when exporting svg with c++ by other process?

Hi:

     I used the following codes to export svg file. It went well. But I want to auto set size unit from inch to mm when showing Export Dialog. Does anyone know how to do that? Thanks!

VGCore::ICorelExportFilterPtr filter = doc->ExportEx(typeUtils::qStringToBstr(tmpSvgFilename), VGCore::cdrSVG, range, opt, pal);

if (filter->HasDialog)
{
    int parentWinId = 0;
    if (params.contains("parent_winid"))
    parentWinId = params["parent_winid"].toInt();
    if (filter->ShowDialog(parentWinId))
    {
        hr = filter->Finish();
        if (FAILED(hr))
        {
            qDebug() << hr;
            QString errorMessage = QString::fromLocal8Bit(std::system_category().message(hr).c_str());
            qWarning() << errorMessage;
        }
    }
}