Parenting methods...

in example parenting Ball_1 to RoundCube_1, select and right click target child ball entry, choose cube as parent

or

drag and drop ball onto cube in Hierarchy which will parent the ball to the cube

or

With desired reparent object selected Object Menu:Change Parent, select target.

You'll probably want your hair prop to load as a smart prop, which will cause the prop to load already parented to its target head. When you save the prop to the Library and it's parented to something while saving, an option to save as a Smart Prop will pop up...

If you like text editing here's the entry for changing something to a Smart Prop in the pp2 controls branch, in this instance the smart parent is the head... change from parent UNIVERSE to the entry shown...