There are two ways to get it. If you talk to her about becoming her lab assistant and do a few quests on the bounty board, you can unlock the TF pills (transformation pills). With this, you can get a penis (you can also cheat and rush a bit of the quest or even give yourself a pill but you won't really know what it will do unless you discovered it to the lab). If you don't want to go to the process, you can mod the game and add Body Modification to your game and then ask her to do an operation to change whatever you need (downsides are costs and the debuff after).
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You seem to be a knowledgeable person for this game, so I'll ask a follow-up if you don't mind. I found the pill, and I just got left wondering if it's possible to have both genitalia at once, since the pill I found replaces the vagina with a penis. It's what I wanted, yes, but given that this is a fantasy game, I wanted to know if it's possible to have both.
I doesn't make sense from a biological perspective, but the same can already be said about other things in the game. I am entirely content with the way things are now, but it could be better, you know? Thanks for reading all of this, and have a good day.