As the above said, think of the move and the slamming separately, because they are.
Make sure your slamming system is 100% stable first. One of the things it must do is hold the target properly (and release them back to a normal state if the slam is interrupted - this is sometimes forgotten and very important). I assume Bloddbane's (which you are probably using) does these things, but I have my own so I can't say for sure. Most certainly you have to use it right.
Once that is down, it's only a matter of triggering the move that has the superslam. No different than any other move. Cancel is probably the better choice, because Cancel can and does take MP into account. You can't cancel into a freespecial if it costs MP and you don't have enough.
DC