The noun dickens (lower case d) has no possessive form, it is a word used for emphasizing a question to express surprise or anger (Who the dickens was that?). To some, using the noun dickens is a euphemism for invoking the devil.
The possessive form for the proper noun Dickens is Dickens's. The possessive of all English singular nouns is formed by adding -'s. Singular nouns already ending in -s acquire a new syllable, along with the -'s of the possessive.
The singular possessive form is week's.
The singular possessive form is classmate's.
The possessive form is Amos's.
The singular possessive form for the noun gypsy is gypsy's.
The singular possessive form for country is county's.
The singular form for the plural noun countries is country; the singular possessive form is country's.
singular possessive: boy'splural possessive: boys'
The singular possessive form is speculum's.
The singular possessive form is aircraft's.
The singular possessive form is frontiersman's.
The singular possessive form is contralto's.
The singular possessive form is herd's.