See this example properly formatted in the QUT Cite tool.
Standard in-text citation examples are in Getting started - In-text citation.
Name of the Treaty, opened for signature Mon. dd, year, volume U.N.T.S. page
Many international treaties are published in the United Nations Treaty Series. Searching for the name of the treaty with 'UNTS' should find you the UNTS volume and page number and the signing date.
Legal references in AMA11 style are based on The Bluebook, and the above is based on Bluebook Rule 21.