Personally I wouldn't see this as any different to measuring out your HE of soya milk and then synning some chocolate drink powder to add to it. So I would probably count it as my hex plus however many extra syns there are over what the plain would be. So for example (picking random figures out of the air, you'd need to look them up):
If 250ml of plain soya milk is a hexa and would have 6 syns if not taken as a hex, and 250ml of the choc flavour is 10 1/2 syns, I'd count it as one hexa and 4 1/2 syns.
But that's just what I would do - it may not be a great idea and others may disagree!
or perhaps look to see if it had more sugar and work out the syns for that and a teaspoon or so of cocoa powder and see the syns for that, Plumfoodie's calculations are easier than mine - I've been known to go the long way round before