Who can initiate an optional referendum at the national level?

Korea, Republic of
CONSTITUTION OF THE REPUBLIC OF KOREA
(Adopted1948; last amended 1987)
http://www.ccourt.go.kr/home/att_file/download/Constitution_of_the_Republic_of_Korea.pdf
(Originaltext: http://www.law.go.kr/lsInfoP.do?lsiSeq=61603)
Chapter IV. The Executive
Section 1. The President
Article 72
The President may submit importantpolicies relating to diplomacy, national defense, unification, and othermatters relating to the national destiny to a national referendum if he deemsit necessary.