Part 1

1. create DiscountItemsEventHandler::SetCustomQuery method
2. add WHERE filter to the ItemName calculated field (e.g. $object->addFilter('test_filter', 'ItemName LIKE ' . $this->Conn->qstr('z'));)
3. go to E-commerce → Discounts & Certificates section
4. create a discount with 1 product
5. confirm, that applied before filter works as expected

Part 2

1. create DiscountItemsEventHandler::SetCustomQuery method
2. add HAVING filter to the ItemName calculated field (e.g. $object->addFilter('test_filter', 'ItemName LIKE ' . $this->Conn->qstr('z'), kDBList::HAVING_FILTER);)
3. go to E-commerce → Discounts & Certificates section
4. create a discount with 1 product
5. confirm, that applied before filter works as expected