MLA

Whitman, Walt, 1819-1892. Johnstone, John to [Edward] Dowden. Box 2, folder 99. 1898 Jun 3. https://collections.library.yale.edu/catalog/2053733.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 2, folder 99"])(1898 Jun 3)[Johnstone, John to [Edward] Dowden]. https://collections.library.yale.edu/catalog/2053733.