It certainly can be bootstrapped by PoW, but doesn't need to be (and usually isn't in practice). The initial stake if it's not bootstrapped by a different consensus mechanism is simply created (not too differently than PoW creates coins). You have an initial block that anyone can create (but in practice, it's generally the creators of the coin who do that), and then the coin created in that and subsequent blocks is distributed in some way (sale, give aways, etc). The fairness of the distribution is always something highly scrutinized by the larger cryptocurrency community, and many coins (pow and pos) have massively unfair distributions of their currency (ie to themselves as a money making or "funding" scheme).