Scan will automatically detect a Snapcode or QR code when one is visible in your Snapchat Camera. If a Snapcode or QR code is detected, Scan will show a banner at the top of the screen asking if you want to unlock the code to add a friend, open a Lens, or access a website. You can access the experience by tapping the banner.
On some older devices, you will need to first take a Snap of the Snapcode or QR code before it is detected.