The public survey page carries a checkbox offering to receive marketing emails and text messages from you. It is a genuinely good place to ask — somebody who has just told you they are happy is the most likely person to say yes.
It only ever opts people in
Ticking it subscribes them. Leaving it unticked changes nothing — it never opts anybody out, and it never overwrites a preference they already had. Somebody who previously unsubscribed and now ticks the box is resubscribed, and their SMS consent is recorded as coming from a survey.
It only counts if you know who they are
The practical consequence is that a requested or automated survey collects consent reliably, and a poster with a QR code often does not. If gathering consent is the point, send the survey to named contacts.
New contacts
Somebody answering a public survey and giving an email you have never seen becomes a new contact, attached to the relevant tribe. That is how a survey on a receipt can build your database.
Say what they are agreeing to
The checkbox names your organisation but nothing else. Use the intro message to say what you will send and how often — it is fairer, and it makes people more likely to tick it.