Cite a work according to what you see on its title page. In this case use the abbreviation “supp.” for “supplement” in the Number element of the works-cited-list entry. The following provides examples of a journal and a book that use the term “supplement”:

Works Cited

Álvarez Gila, Oscar, et al. “Western Sahara: Migration, Exile and Environment.” International Migration, vol. 49, supp. s1, 2011, pp. e146–e163.

W. H. Auden to O. Henry. Scribner, 1974. Supp. 2, pt. 1 of American Writers: A Collection of Literary Biographies.

The entry for the journal article by Álvarez Gila et al. uses the abbreviation “supp.” in the Number element. The entry for the book W. H. Auden to O. Henry uses the abbreviations “supp.” and “pt.” as part of the supplemental element for a multivolume work.