[tricky]

Не знаю, корректно это или нет, но у меня сработало вот так

    PurchIdBase purchId = 'ПЗ0050395';

    PurchTable  purchTable = PurchTable::find(purchId);
    PurchLine   purchLine  = PurchLine::find(purchId, 3);
    Args        args = new Args();
    FormRun     formRun;
    FormDataSource fds; // добавил
    ;
    args.name(formstr(PurchLine));
    //args.record(purchLine);  // закомментил

    formRun = classFactory.formRunClass(args);

    formRun.init();
    //-------- добавил -----
    fds = formRun.dataSource();
    fds.query().dataSourceTable(tablenum(PurchLine)).addDynalink(fieldNum(PurchLine,PurchId),purchLine,fieldNum(PurchLine,PurchId));
    fds.query().dataSourceTable(tablenum(PurchLine)).addDynalink(fieldNum(PurchLine,LineNum),purchLine,fieldNum(PurchLine,LineNum));
    //------------------------
    formRun.run();
    formRun.wait();