Both Gen 1 Decora Smart® products with HomeKit Technology (DH1KD, DH6HD, DH15S) and Gen 2 Decora Smart Wi-Fi products can be learned into the Apple Home app even if you lose the manual or box with the QR code.
During the traditional Add Accessory process a QR code on the box or manual is scanned by the app. If you are learning a Gen 2 Decora Smart Wi-Fi product a QR code can be found in the lower right corner. Simply remove the wall plate, scan the code and replace the wall plate.
If you are learning a Gen 1 Decora Smart product with Homekit Technology (DH1KD, DH6HD, DH15S) or if your Gen 2 code will not scan you will need to use the HomeKit setup code. Choose the 'Use the HomeKit Setup Code' option and enter the code found in the lower right corner of the product. Simply remove the wall plate, choose the option , type in the 8 digit code and replace the wall plate.