On the Topic Manager page, click on "+ new challenge".
Scroll down, click on the QR Code Challenge.
Create the Challenge as per usual, by filling out the fields.
Now, go ahead and click 'save challenge'. Your QR Code will be generated after you post your challenge.
After creating this QR Code Challenge, you will be brought back to the Topic Manager page. Scroll to the QR Code Challenge you have just created and click on "Get QR Code".
Your QR Code is now available for download.