its not a mask. you should have completed the quest for sayla when you go to get her rare north yellow leaves from the north, you get captured by ull and imprisoned and have to escape the caves and get back to sayla where shell add the crafting item in your consumable/food menu (left on d-pad for xbox and i believe the same for ps4) and you can auto select y as the primary thing you want to use. or when you get close to the poison go back to that menu and choose to use it (believe it requires meat and obviously rare north yellow leaves and should be on the second row)