[Dumfag]

Пропиши в методе dialog:
rContractCodeField .registerOverrideMethod(methodStr(formStringControl, lookup), methodStr(Название класса, rContractCodeField_lookup), this);

protected Object dialog(DialogRunbase _dialog, boolean _forceOnClient)
{;
    dialog = super(_dialog, _forceOnClient);
    dialog.caption("Задайте параметры");

    rContractCodeField = dialog.addFieldValue(TypeId(RContractCode), rContractCode);
    rContractCodeField .registerOverrideMethod(methodStr(formStringControl, lookup), methodStr(name_class_from, rContractCodeField_lookup), this);
    return dialog;
}