When someone has a last name that ends in and "s", what's the correct way to show possession?

For example, would it be:

Birkerts' essay
Birkerts essay
Birkerts's essay

??

I'm supposed to be writing a paper about an essay by Birkerts.

If you would pronounce it "Birkertsez," type s's. If you would pronounce it "Birkerts," use s'. My choice would be s-apostrophe.

Thank you! I knew you would know the answer! :)

Thank you! I knew you would know the answer! :)
You're welcome. I'm an expert on a grammar site. :D