Use the guidance on Letters (from an online archive or collection) to reference letters found in onine archives.
You will need:
Author Surname, Initials (Year) Letter information eg. sender and recipient, Day Month. [Letter]. Place of Archive: Name of Archive. Catalogue/ reference number.
Wall, A (1909) A. Wall, Hon. Sec., to Archdeacon Wright, 7 May. [Letter] Manchester: Manchester Archives and Local Studies. Catalogue number: M383/4/3/1-3
In-text citation: (Wall, 1909)