Legal provisions for optional referendums at national level

Belarus
Constitution of Belarus
Constitution 2004
https://www.constituteproject.org/constitution/Belarus_2004.pdf
Article 73
National and local referenda may be held to resolve the most important issues of the State and society.
Article 76
Referenda shall be conducted by means of universal, free, equal and secret ballot. Citizens of the Republic of Belarus eligible to vote shall take part in referenda.
Article 77
The decisions adopted by referendum may be reversed or amended only by means of another referendum, unless otherwise specified by the referendum.
Article 78
The procedure governing the conduct of national and local referenda and a list of issues that may not be put to a referendum shall be determined by the law of the Republic of Belarus.
(Optional and Mandatory referendum)
Article 140
The Constitution, laws on amendments and addenda thereto, on the entry into force of the said laws and instruments on the interpretation of the Constitution shall be deemed to have been adopted where no less than two-thirds of the elected deputies of both chambers of the Parliament have voted in favour of them.
The Constitution may be amended or supplemented via a referendum. A decision to amend or supplement the Constitution by means of a referendum shall be deemed adopted where a majority of citizens on the electoral roll have voted in favour of it.
s 1, 2, 4, 8 of the Constitution may be reconsidered only by means of a referendum.