You want to send a QR code to contacts so that you can use the QR code scanner of the check-in app?
Then go to the message in which you want to send the QR code and open the placeholder manager. Now click on "Contact information" and select "QR code" at the bottom:
This is what your email will look like:
And here's what the contact will receive after sending:
For a greener world
You can actively inform the participants in the email that the QR code does not have to be printed out. The QR code can be scanned directly from the contact's smartphone.