Discounts are available for Shopify Post Purchase offers only. Upsell does not have a feature to discount products for before checkout offers.
Before checkout offers can be discounted by integrating Upsell with Bold Discounts. This integration allows you to create a hidden duplicate product with a discount. This duplicate product appears only in the Upsell offer window and not on your storefront. For more information, please visit Upsell & Bold Discounts Integration.
Note: If Bold Discounts is not an option for you, buy one, get one (BOGO) offers can be achieved by using Bold Bundles. Please visit Create/Edit/Delete a Bundle for instructions on how to create a BOGO bundle.