What is one of the rules for forming the possessive case of words that end with an S or s sound?

There are two accepted forms for possessive singular nouns that end with an s:

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

Example: "Put it on the boss' desk."

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

Example. "Put it on the boss's desk."