do it at transplant so you can put it where it'll work best. when you originally sowed seeds in smaller pots is also a good time to do it. that biote needs roots to survive.
Follow instructions relative to size of the pot... products vary in spore count. you may need more or less depending on potency of inoculation and definitely proportional to volume of substrate invovled. So, a little pot for your seedlngs, thena bit more than previously for transplant.