MLA

Whitman, Walt, 1819-1892. Kennedy, William Sloane to [Laurens?] Maynard. Box 2, folder 101. 1897 Jun 10. https://collections.library.yale.edu/catalog/2053735.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 101"])(1897 Jun 10)[Kennedy, William Sloane to [Laurens?] Maynard]. https://collections.library.yale.edu/catalog/2053735.