Hidden fields and pre-filled answers

Give a question a key in its settings, say source. A link with ?source=newsletter fills that question in for the person answering.

#Hidden fields

Add a Hidden field from the Smart group. People never see it, but its value is saved with the response: a campaign name, a customer ID, a referral code. Set a default value for when the link has none.

#Pre-filling visible questions

Any question with a key can be pre-filled the same way. The person sees the answer and can change it.