United Nations treaty
Treaty title. Year signed. [Online]. Volume number and abbreviated Treaty Series name and 1st page number, opened for signature/adopted full date, and entered into force full date. [Date accessed]. Available from: URL
Example:
Convention on the Rights of Persons with Disabilities. 2007. [Online]. 2515 UNTS 3, opened for signature 30 March 2007, entered into force 3 May 2008. [Accessed 25 August 2017]. Available from: https://treaties.un.org/doc/publication/UNTS/Volume%202515/v2515.pdf
The authoritative source for UN treaties is the UN Treaty Collection. This should provide all of the details required for a full reference.