I tried for myself, you have to beat the second dungeon and then talk Freya. Once you do, you get the option to "exchange 3 vials of cum", otherwise it doesn't show up despite having the vials of cum. This is probably done to avoid spoilers, or this is probably where the second wave of secondary tasks are unlocked
This will probably lead to a lot of people missing it, since people are told about the vials and the eau of cum way earlier :/