Have just purchased ESO PLUS MEMBERSHIP through steam, but it was not added to the game. I got only bonus crowns, but not a single character bonus or item (craft bag) from eso+m.
PS membership is marked as "not active" in game.
Steam redirects here
"Unfortunately, The Elder Scrolls Online: Plus Membership does not utilize Steam to manage its items. You will need to contact their Support Team for any questions about your items in that game. "