You purchase it in-game via the Crown Store. There should be an option for it, which will take you to the Xbox Marketplace to purchase it through Xbox, not ZOS/ESO directly.
If you go in the crown store, on the bottom right hand corner, it says to click in the right stick of your controller to purchase ESO Plus, know that you can only get a one month subscription on Xbox One. That should answer your problem.
If you see me anywhere. Know that I am sitting back with a bag of popcorn, watching as ESO burns the goodwill of its player base with practices that only disrespects the players time like it did to me and many others...
If a game does not respect your time, best thing to do is move on from it and find something else.