tModLoader Trying to make spawn conditions for a custom modded NPC. Can anyone help?

[LogLog]

Hello! I've been looking at some youtube tutorials on how to create custom things for a tModLoader mod, but I've hit a roadblock here. I made a custom NPC, but I don't know how to set its spawn conditions?
Basically, I want this NPC to only move into your town if you have another certain NPC that's already moved in. Is that possible, and if so, how would i do something like that?

 

[Not is Many]

See terraria source? See Example mod? Set if some Npc exists (NPC.active = true)?