While, I've not added candi sugar to the fermenter, I have added regular sugar and I imagine the process would be similar. I would simply boil the candi sugar in a small amount of water, let it cool, spray some star san around the outside of the pot and carboy opening, then dump it in.
For what it's worth, I've heard the hard, rock candi, sugar sold as Belgian candi sugar is simply expensive sugar. You may want to consider using either regular sugar, the dark candi syrup, like this
, or making your own candi sugar.