The publisher’s support site states that if you’re among those eligible for refunds, you won’t need to do anything as Ubisoft will reach out to you on its own regarding the refund. Do note that you’ll only be eligible for a refund if you bought the PC version of the game after 15 April.
In case you missed the news, Ubisoft’s reasoning behind giving away Assassin’s Creed Unity for free is so that people can still visit the world famous landmark through the game which was set in Paris during the French Revolution period. As a quick reminder, you should claim the free game if you haven’t done so already. The offer is only available for the PC version of Assassin’s Creed Unity and can be obtained through Uplay until 3:00 PM on 25 April 2019. (Source: Ubisoft via PCGamesN)