How to use possessive form for proper nouns that already have an apostrophe

How would one write a possessive form for a proper noun that already has an apostrophe-s in it? For example, I want to use the possessive form of "McDonald's" maybe in the sentence "The Market Street McDonald's' employees are friendly."

I realize the possessive form may not be entirely necessary there, but in a case where it is, what is correct?