Who can initiate an optional referendum at the national level?

Lesotho
Answer
King, on the advice of the Prime Minister
Source
Constitution 1993 (last amended in 2011)
Article 84B.
1. The King may, on the advice of the Prime Minister order that a referendum be conducted to obtain the opinion of the electors on any matter he considers to be of national interest.
2. Parliament may make provision for the conduct of a referendum.