MLA

Cullen, Countee, 1903-1946. Hughes, Langston. Box 1, folder 18. 1928 Apr 4. https://collections.library.yale.edu/catalog/16731064.

APA, 6th edition

Cullen, Countee, 1903-1946. (["Box 1, folder 18"])(1928 Apr 4)[Hughes, Langston]. https://collections.library.yale.edu/catalog/16731064.