Q. How do I create a citation and reference for the WHO constitution using the APA Publication Manual, 7th ed.
There's a special rule for constitutions in the APA Publication Manual, 7th ed. The rule is not in the Reference Examples chapter (chapter 10), it's in Legal References, chapter 11. The section on constitutions is 11.9 on pages 366-368. I suggest that you follow the example given for the U.N. Charter since both are international legal documents.
If you are citing the entire constitution, the introductory paragraphs to the section say that you only have to refer to it in the text, you don't need to create a citation and reference.
If you are citing a specific part of the constitution, then follow the Example for the reference and the Parenthetical citation example for the parenthetical citation. A parenthetical citation and the reference will be the same, except the reference will not have parentheses around it. There is also a separate example for creating a narrative citation.