See this example properly formatted in the QUT Cite tool.
The Convention was implemented to protect the rights of children (United Nations Convention on the Rights of the Child, 1989).
The United Nations Convention on the Rights of the Child (1989) was the most ratified UN Convention …
Name of Treaty or Convention, Signing/Approval Month Day, Year, URL if available
United Nations Convention on the Rights of the Child, November 20, 1989, https://www.ohchr.org/en/professionalinterest/pages/crc.aspx
The format for a Treaty in APA is based on the Bluebook.