Who can initiate an optional referendum at the national level?

Netherlands: Advisory Referendum Act, [Dutch version]; [Regulations expire on 12-07-2018 with retroactive effect up to and including 10-07-2018], accessed 11 November 2018

NLTimes: DUTCH SENATE APPROVES SCRAPPING ADVISORY REFERENDUM; By Janene Pieters on July 11, 2018, accessed 11 November 2018

Act of 30 September 2014, containing rules concerning the advisory referendum (Advisory Referendum Act) has been abolished by the Senate on 11 July 2018.