How about adding 's to the name of a company that ends in s for a possessive? Is it the same guideline (go by how it would be pronounced)? Thanks.
In U.S. English, this depends largely on the style guide in use. When a single entity ending in "s" possesses something--say, Toys "R" Us--it can be written as
Toys "R" Us' products
Toys "R" Us's products
depending on whether you are using an AP-Stylebook-based approach (source, source) or a Chicago-based approach (Sec. 7.18, 15th ed.). If no one has specified style guidelines for you, either way is fine as long as you stay consistent.
Grammar Girl discusses this at some length here and agrees either way is correct.
She also discusses the McDonald's (McDonald's'? McDonald's's?) conundrum here. She says the same thing I would--find a way to rephrase!