Swift segue to xib file programatically

I cant seem to figure this out. I am new to swift. i need to load a xib file from a navigation bar button item. I have to do this outside the storyboard because the same button item changes depending if the user is logged in or not. It reads "Login/Register" if a user is not logged in and goes to a login xib file and if a user is logged in it reads "Add Story" and goes to a add story xib file. Thank you for your time I really appreciate it.