What is the correct spelling of the possessive form of Morris?

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

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

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

EXAMPLES

Mr. Morris' office is on the third floor.

Mr. Morris's office is on the third floor.