If you have bought your ESO on Steam (or linked your account to your Steam account), and you have ESO+, you might have noticed the game thinks you don’t have it active.
We are currently investigating reports of #ESO Plus benefits not being granted on PC/Mac. Thank you for your patience.
— Elder Scrolls Online (@TESOnline) January 18, 2017
Yeah whatever, it’s been way more than 24 hours now and no response.
You can play the game with ESO+ if you launch it from its executive file (and not the launcher), which most likely will be in your “\Steam\steamapps\common\Zenimax Online\The Elder Scrolls Online\game\client” folder. If you don’t know where that is, just search for eso.exe or eso64.exe (depends what type of OS you have, but most likely it’s 64-bit. I’ll let you google how to find out if your OS is 32 or 64 bit tho).
You can also just add the game to Steam if you need the overlay. Go to Library, below the list choose +Add a game, then browse and look for the file mentioned above.
It won’t log in automatically for you (as it skips the Steam link, to avoid the ESO+ mess that is right now), but you will be able to play the game with the overlay. It also won’t display any game related Steam links on the overlay, as it won’t be recognized as ESO.