Who can initiate an optional referendum at the national level?

Article 82.
The President of the Republic on the proposal of the Government during the sessions or on a proposal from the National Assembly published in the Official Journal and on the advice of the Constitutional Council, submit to a referendum any bill on the organization of government, with approval a union agreement or authorizing the ratification of a treaty which, without being contrary to the Constitution, would affect the functioning of the institutions.

Optional referendums are initiated by the President, according to Article 82 of the Constitution.