(This applies to NPCs as well as your own character, half way into writing this I think I understood what you were asking. If you already know this, just skip down to the bottom.)
You need to have a penis or clit that has the ovipositor modifier in order to lay eggs in people, does not matter the penis type. But you must have a egg laying vagina that is pregnant to do it. If You want to do it the legit way, take any of the species foods and enchant it, go to the penis or vagina tab and give it oviposition, then drink it and now you have the capability to lay egg during sex instead of creampie, choose lay eggs.
For NPC however, they decide on their own whether or not to lay eggs in you, you cant tell them to do it or not. The game just uses the same "ask them to cum inside or pullout", laying eggs is one of the cum inside option they might use if they accept your request. As a tip, I reccomend that you dont give the egg laying character extra ways to cum (like adding a knot to their penis), the more options they have to cum inside the less likely egg laying is picked. if they cum inside your vagina and you get pregnant from it, they can no longer lay eggs in there.
