What is the possessive of Father Jess?

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: Father Jess'

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

Examples:

We're planning a party for Father Jess' birthday.

We're planning a party for Father Jess's birthday.