Note: Dynamic bones don't always work, if the hair is composed of multiple components, sometimes those components aren't good enough (won't have enough nodes for dynamic bones) or they're simply bad placed, so you may need to edit the character. If the whole hair item is modded in and dynamic bones doesn't work as well, you'll have to either change it or deal with it by editing (rotating/scaling/moving) the bone of the accessory itself, not ideal). Also look into FK.