I think I understand. In my last post I thought you meant you were entering your homemade random number into their website. Which seemed to me wouldn’t work.
I now think you meant that you write your homemade random number on your receipt from this visit as the “proof” that you did the survey. Then you redeem this survey result for your next meal. They don’t actually check the number beyond those last 2 digits before giving you credit on this new visit. Lather rinse repeat.
Which reminds me of this post of mine from a few years ago:
Which, funny enough was started by an OP about McD’s lax coupon validation.