Most games come packaged with one or more cards containing codes these days and controllers aren't the smootest type interfaces ever made but with Xbox One you'll be able to scan in a code using the Kinect sensor.
In a Vine post released by Larry 'Major Nelson' Hryb the Xbox PR frontman demonstrates the process in action. Assuming everything works as smoothly as it does in the video once the console is released it should be pretty straightforward to redeem content codes on Xbox One.
All you need to do is say, "Xbox, use a code" and hold the QR towards the Kinect sensor's camera.
The Xbox One is set for release on November 22nd though not all Kinect capabilities will be available in each of the 13 launch territories – eight countries will not have the "Xbox on" voice command option from standby mode initially and only ten of the 13 will have "some voice features".