It sure isn't going to live well or long indoors, so I'd plant out ASAP. It needs shade or at least part shade and well-drained soil. IME, Japanese anemones tolerate some rather poor soil conditions as long as drainage is good. Better, fertile soil will allow for greater drought tolerance - poorer, drier soils will necessitate more routine watering.

fyi, some varieties of these anemones tend to spread rather vigorously.

Here is a link that might be useful: how to grow Japanese anemones