MLA

Whitman, Walt, 1819-1892. Kennedy, William Sloane (1850-1929). Box 2, folder 83. 1884-89, n.d.. https://collections.library.yale.edu/catalog/2053717.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 83"])(1884-89, n.d.)[Kennedy, William Sloane (1850-1929)]. https://collections.library.yale.edu/catalog/2053717.