What is the possessive form for deaconess?

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the Word: deaconess'

Add an apostrophe s ('s) after the existing s at the end of the Word: deaconess's

Examples:

The deaconess' appointment is at six.

The deaconess's appointment is at six.