You need an operative with a unique weapon - not one that you purchase from the online store (Thanks Gnostic1974)
Open the menu and head to your team. Select the operative with a unique weapon and open their edit page. When you select their weapon to edit, you’ll have another menu with the skins. Buy any one of them and the trophy will drop.
KTKarona has confirmed that this can actually be done with any of the non-lethal weapons obtained from the skills upgrade, making it easier to obtain the trophy!