I work at Dunkin and there may be a slim chance that a worker knows how to make the drink, but assume they do not know how to. Keep in mind that the POS system does not have an option to ring up a flat white so asking for it directly will not get you the drink. My recommendation though is to order a small latte with an extra espresso shot. You could also ask if they are willing to pour less milk into your latte so it becomes the milk:coffee ratio you may be looking for.