I don't think this has yet been covered in any of the other questions on similar topics. There was one other very similar question, however, it was not specifically talking about the case where the proper noun ends in a plural noun. Feel free to vote to close if I am wrong.
What is the correct way to make a proper noun ending in a plural noun ending in an "s" possessive? This frequently happens with corporations, e.g., "Dunkin' Donuts." Should one work off of the fact that the entity is singular—suggesting Dunkin' Donuts's—or should one work off of the fact that "Donuts" is plural and ending in an "s"—suggesting Dunkin' Donuts'?
I expect that the answer might depend on dialect because some regions refer to corporations in the singular form ("Dunkin' Donuts is a company") while other regions refer to corporations in the plural form ("Dunkin' Donuts are a company"). I am specifically interested in American English, but would be interested in hearing answers for other dialects too.