Does anyone know how to get the birthday suit outfit or the coding for it to add it on your profile?

Never heard of it, where'd you get the information that it exists?

I saw it mentioned in a thread a page or two back a while ago. Is there not one? 

https://itch.io/t/1969549/outfit-guide

this one, it hasn’t had any replies though and is kind of old.

Ok new update just came out and it actually exists, not only that but it existed before the update. I have no idea how to get it, but it most likely has something to do with Urka scenes. Urka being the symbol of the game it also can be connected to some another major encounter. Probably Monster Contest in the last level. Gonna check it

are you talking about the new years brothel madame picture?

It is the naked urka when you look at her when she gets out of the water when you visit her at the camp. She is naked.