I built a small solution for this that others here might find useful, so sharing in case it saves someone the trouble of building it themselves.
LimeSurvey's own End URL setting already supports everything needed — no plugin required                                                                                                          - For a token-based survey, add email={TOKEN:EMAIL} to your End URL and the participant's email carries straight through.
  - For an anonymous survey, add an email question and reference it the same way.From there you just need something on the receiving end that (a) verifies the request is genuine, (b) assigns the next available reward code, and (c) emails it — and does it idempotently, so a reload or resubmission never issues a second code.  I wrote exactly that as a small standalone service (REWARDR) since I needed it for a study I run myself — happy to share details if useful: . Not trying to spam the thread, just leaving this here since I searched for exactly this a while back and found nothing ready-made.

Seems interesting, but how do you disable such behaviour :

• I launch a survey, and I'm finally redirected to your website.
• Then, I create a bunch of emails that directly go to your website without doing the survey.
• A lot of money for me, and nothing for your client
• Your client loses money, you lose your client

PS: I disabled the directlink, I ask to administrator if we can leave it