Apostrophe after first use of acronym [duplicate]

It's a matter of style.

If you are writing as an employee of an organization, you may have a style guide that dictates such constructions. As an editor for The Boeing Company, I have a rather thick style guide that directs how we use acronyms; as a rule, we do not use possessives with acronyms - in fact, we avoid using possessives as much as possible.

If this is a personal letter, I would suggest that you define your first instance of the acronym simply, without the possessive form. You can treat subsequent uses of the acronym as nouns, meaning they could take the possessive.