Having little experience with candi sugar I have only put it in the boil. I have a hard time seeing it dissolve very well added to the fermenter, there about peanut size chunks.
Agreed, it could be tough to ensure it dissolves in the primary if added dry.
You could add the candi sugar to 2 cups of boiling water, dissolve the sugar and boil for ~10 minutes (basically making a syrup), then take off the heat, cover and cool to room temp, then dump into the 1°.
Just a thought.