When citing letters that are suspected or confirmed forgeries, you can indicate in the Author element of your works-cited-list entry both the name of the author as shown in the letter as well as the attributed author. Place the attributed author’s name in square brackets. If the forgery is not confirmed, you may want to include a question mark.
Peale, Rembrandt [Ferdinand Danton, Jr.?]. Letter to Peter Roller. 5 April 1851. Washington College, Archives and Special Collections, MS 0116, washcoll.whirlihost.com/Detail/objects/6724.
For additional guidance, see our post on disputed authorship.