Props do not conform. Only characters (which can also be hats and clothes) can do that. Some props are smart props, meaning they go where they're supposed to, fo r instance on the figures head if it's a hat. But that seems not to be the case here. You will have to position it manually and then parent it to the head once it's in place.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.