I know we chatted separately, but adding my overview answer here in case others come by here…

There are no 'QR Code' APIs. What I have done in the past was create QR codes using a service like this: https://qr.io/api-documentation

Have done this two ways:

1. Set up the events to mark complete on launch. Then you set the QR Code URL to the session URL. When they put them up on the screen, the user just has to follow it, join and get credit. 
2. The URL in the QR code points to a custom page and you add the Session ID to the link. Using an embedded form on the page, once the user logs in the form basically collects the users ID and the Session/Event ID in the background, and presents itself as a 'I affirm that I attended this event' form. Once they submit, it kicks off a workflow to use the update Session/Event with a completion for the user. This one takes more setup, but is far more flexible and more accurate. 

I go over some of this in this article: https://community.docebo.com/integrations-65/you-built-it-they-came-now-what-scheduling-ilt-wizard-bonus-activities-7496#Instructor+Invite+(Adding+a+QR+Code)

If folks are interested, like this comment, and I can do some more in depth write ups on this specifically.