Who can initiate an optional referendum at the national level?

Wetraadgevend referendum (Consultative Referendum Law) 13-07-2015

http://wetten.overheid.nl/BWBR0036443/geldigheidsdatum_13-07-2015#Hoofdstuk7_Paragraaf2_Artikel44

See also: https://www.kiesraad.nl/artikel/raadgevend-referendum

If sufficientnumber of voters submit valid requests, a consultative referendum can be held.Submission of requests is done in two stages: preliminary request and finalrequest. For the preliminary phase valid request of at least 10,000 voters is needed.The Electoral Council publishes the decision on the phase of the preliminaryrequest in the Gazette. Afterwards voters have six weeks to submit a finalrequest for holding a referendum. This requires 300,000 requests.