At the show
Open the QR while it is on screen. Type the code here if you need to.
What if the QR is already down?
The chance is gone. Scan while that QR is on screen.
Trouble scanning?
The QR is a link for this show — while it is on screen. The QR is a link another phone can open. Use the phone camera on it, or type the code. Scan now follows the QR on screen. A saved link dies when the QR comes down. A remembered code after they take it down is gone.