There is option only if you have application installed but it can't be played live on computer!
It would be great to have this possibility.
at the very bottom of this page you will see " Try the new Chess.com!" , you can click that and find 960 games . /click the link above if its active , if not then go to the bottom of this page.
ETA: If you mean to play a chess960 computer then the only way to do that is to use a random chess generator and a online chess board editor , plug in the pieces from the random chess generator into the board editor and play ..... problem solve // i know it works because i've been playing that way from last year