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Creatine works via muscle saturation brehette. It is neither a "pre" or "post" workout supplement, it just needs to get there. It doesn't really matter. Uptake potential will be higher post but that's kinda irrelevant...
If you're already taking a PWO shake, then add it to the shake and you're gtg. You should only need 3 grams unless you're a biggums.
(Also, don't bother with a "loading" phase in which you take more than a normal daily dosage for a week or two to speed up the saturation timeline; you can only absorb so much and the rest will just get peed out - literally.)