Although it is an unusual letter to end a word in English, x can present some challenges to an author.
With a word like box, the possessive version is fairly straightforward: the box’s hinges.
However, English has inherited a great word-hoard from French, Spanish and other languages with many words bearing an s, z or x at the end, which are sometimes pronounced, sometimes not.
Personal names are a case in point, as people tend to have possessions and possessions generally demand apostrophes.
As a rule of thumb, adding the s to the apostrophe is a good idea: for example, the chateaux’s imposing walls or Marx’s impressive beard.
In the past adding an s may have seemed very clumsy. However, style guides are now pointing in that direction, even ones in the US where ideas about grammar tend to be more traditional.
Names from other languages can sound a little awkward in the possessive form when spoken out loud, but adding that s seems to be the safest and clearest course.