If users with iPhones already have Code Scanner in their Control Center:
On an iPhone with Face ID: Swipe down from the top-right corner of the screen and click Code Scanner
On an iPhone with a Home button: Swipe up from the bottom edge of the screen and click Code Scanner
If users do not yet have Code Scanner in their iPhone's Control Center, they can add it quickly for free. Here's how:
Go to Settings
Go to Control Center
Tap the green circle with the white + next to Code Scanner
Code Scanner icon appeared in the Control Center