MLA
Porter, John Addison, 1856-1900. Letter from John Addison Porter, Secretary to United States President William McKinley (Washington, D.C.) to William E. Mickle Jr. regarding William's proposed commission to the United States Army - February 2, 1899. February 2, 1899. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/064tht77kb-cor.
APA, 6th edition
Porter, John Addison, 1856-1900. (February 2, 1899)[Letter from John Addison Porter, Secretary to United States President William McKinley (Washington, D.C.) to William E. Mickle Jr. regarding William's proposed commission to the United States Army - February 2, 1899]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/064tht77kb-cor.
Chicago
Porter, John Addison, 1856-1900, Letter from John Addison Porter, Secretary to United States President William McKinley (Washington, D.C.) to William E. Mickle Jr. regarding William's proposed commission to the United States Army - February 2, 1899, February 2, 1899, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/064tht77kb-cor.