Is the singular possessive of mess mess's?

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: mess'

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

Examples:

The note read, "Who is this mess' owner and when will it be eliminated?"

The note read, "Who is this mess's owner and when will it be eliminated?"