To tag a QR Code as a source, you will have to push it as a custom marketing source. To do this, we invite you to use the tracking variables at_ in the URL link contained in the QR Code.
For more information on the technical process of at_ variables:
As a reminder, this one must have at least the variables at_medium and at_campaign.
For example : https://www.monsite.com/?at_medium=QRcode&at_campaign=macampagne