No voucher is needed for this just an NUS card.
The deal is a long standing deal (at least 15 years) that with an NUS card, when you buy any meal you get a free McFlurry, cheeseburger or hamburger.
What isn't well known is that there is a different "wrap of the day" meal each day. It is t even always shown in the kitchen area with all the prices, or isn't obvious. All of them feature chicken. They are very nice, quality wraps, in fact nicer than the buns mcd use on most of the burger range for instance.
So for £3.59 you get:
Chicken wrap (some days also features bacon, sweet chilli sauce etc)
Medium drink, fresh Tropicana Orange juice or coffee or tea
McFlurry (promo ones not included)/