Maintenance for the week of March 25:
• [COMPLETE] ESO Store and Account System for maintenance – March 28, 9:00AM EDT (13:00 UTC) - 12:00PM EDT (16:00 UTC)
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.
"When the human race learns to read the language of symbolism, a great veil will fall from the eyes of men." ~Manly P. Hall