What is the singular possessive of goodness?

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: goodness'

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

Example:

They say that goodness' reward is the goodness itself.

They say that goodness's reward is the goodness itself.