Ok so you need to make sure that armor has the "Riven's Curse" mod on it and you have to be wearing a full set with that mod. While wearing the full set, popping a tincture will make you appear Taken.
I know some of the Dreaming City triumphs are bugged, so if this doesn't work now I'd suggest trying again when Season 13 starts next Tuesday
English
-
I figured out the issue. You can't already be ascendant while putting the armor on. You need to pop a tincture while already wearing the armor. Kind of confusing and arbitrary, but I guess that works(?)