MLA

Williams, William Carlos, 1883-1963. Burke, Kenneth. Box 3, folder 96-99. 1925-1962, undated. https://collections.library.yale.edu/catalog/17049457.

APA, 6th edition

Williams, William Carlos, 1883-1963. (["Box 3, folder 96-99"])(1925-1962, undated)[Burke, Kenneth]. https://collections.library.yale.edu/catalog/17049457.