Table of Contents
1, go to Catalog -> Products section.
2, Click Edit Product.
3, The first field is used to search the desired products you already have in the store and click that product.
PRESTASHOP 1.7.X – HOW TO CREATE A CROSS-SELLING PRODUCT #
https://www.templatetrip.com/help/prestashop-1-7-x-how-to-create-a-cross-selling-product/