How can I populate lookup in Product from Script Task?

It's possible to implement via multi-row data insert. The example for the "Prices" is below.

var priceListId1 = new Guid("FA689C95-C63C-4908-8FD2-19A95E0425BD");

var priceListId2 = new Guid("90699A9B-99AF-49B3-9678-73BDAE7EAEFE");

var priceListId3 = new Guid("7635F047-2DD4-4060-AF07-D7B9820BBC34");

var productId = new Guid("A802F388-59AA-4B69-88C3-ECC9078BF27D");

new Insert(UserConnection)

.Into("ProductPrice")

.Values()

    .Set("PriceListId", Column.Const(priceListId1))

    .Set("ProductId", Column.Const(productId))

.Values()

    .Set("PriceListId", Column.Const(priceListId2))

    .Set("ProductId", Column.Const(productId))

.Values()

    .Set("PriceListId", Column.Const(priceListId3))

    .Set("ProductId", Column.Const(productId))

.Execute();

Please find more information in the article by the link below

https://academy.bpmonline.com/documents/technic-sdk/7-13/multi-row-data-insert