You can use QR codes however you like on your artwork/creatives, however there are some restrictions depending on the formats or types of boards/placements you’re targeting in your campaign.
In general, QR codes can be used on most inventory that is away from the roadside, or otherwise will not be construed as a distraction to drivers. The last thing I want is to be accused of encouraging motorists to whip out their phone to scan a QR code on a billboard - y’feel?
The best approach would be to consider the board(s) you’ve chosen for your campaign and where they’re placed before choosing to add a QR code into your artwork. If your artwork absolutely must have a QR code on it, I’d refrain from any of the outdoor formats (such as billboards).
Lastly, if you’re unsure about a specific board or set of boards - please feel free to reach out to the team and ask them about it.