install ibus-korean on ubuntu software(searchbar appears when you start to type in)
open ibus-setup in terminal
go to input method and add Korean-Hangul(this didn't show initially but appeared after a reboot for me)
go to settings/Region & Language and add input source = Korean(Hangul) 
I suggest you delete whatever English input source you used to use since Korean(Hangul) also has a english input