You can get PP post chargen for 5 karma per point, but still only up to your starting magic rating. Beyond that you need to initiate, raise your magic rating and pick a point of PP instead of another metamagic.
In most cases you would get all your PP at chargen, but sometimes you have more important things to buy so you get the last points post chargen. Like Kyra, the text book Mystic Adept example.
If you buy PP during chargen you would do it at step seven of the chargen process.
You might or might not also be allowed to spend karma to initiate during chargen. Not sure if we ever got clarification on that.