Marketing Cloud: Add QR Code to Email and Update Data Extension Column when code is scanned

My use case is the following: For an event, I want to sent an email with a QR Code to the contacts who registered for the event. I will use a single-send-journey.

All contacts data are stored in a Data Extension. My subscriber key is the contact ID from SalesCloud.

When the QR code is scanned, the following should happen: One column in my data extension should be updated with a new value (Column: EventTracking, Value = Attended)

I think I need to add a code to my Ampscript, but which one?

I managed to create the QR code that shows the Subscriber Key when I scan it.

And I managed to update the Column, but directly when I sent the email (I used: @upsertDE). So both work independently from each other ;-)

Any Idea what I need to do, to update the Data Extension only when the QR code is scanned?