The dealer wouldn’t be giving it to you for free, they would charge motability using the appropriate code and be reimbursed for refilling your car with adblue, so their is no reason for them to refuse or moan about it.
They probably would whinge a bit, can you imagine you go into your local VW dealers and heaven forbid you ask for a discount to match Marshalls (£400) and then you have the sheer cheek to buy it from Marshalls saving yourself a shed load of cash then expect you local dealer to service it and now you are asking for free adblue, shame on you.
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.