You have to log on to the My CASA portal on your phone as the CASA system then recognises your phone. There is something about it on the My CASA website on the computer about downloading a QR code but the only QR code that I can find is on the digital version on my phone. It then joins your Covid certificate in the wallet.
I'm not overly computer savvy so I have more than likely missed something. It also doesn't work on Android yet.