How do you make United States possessive?
The possessive noun form is United States’ (e.g., “the United States’ largest park”). Even though “United States” is treated as a singular noun, “States” itself is plural. So, it follows the rule of adding only an apostrophe to form the possessive.
“United States” is not typically used in its possessive form, though. It is often more natural to use a prepositional phrase (e.g., “the largest park in the United States”).
Additionally, “United States,” or “US,” is often used as an adjective rather than a possessive (e.g., “the US economy”).