What is the correct spelling of the singular possessive form of 'St Louis'?

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; for example:

• "St. Louis' Gateway Arch is a well known landmark."

Add an apostrophe s ('s) after the existing s at the end of the Word; for example:

• "St. Louis's Gateway Arch is a well known landmark."

I personally use the -'s as the example above. If you are a student, use the form that is preferred by your teacher.