How do I make a mannequin/dummy wear a 3d item/accessory/layered clothing?

I am trying to add an accessory to an R15 mannequin, but it is simply moving to the mannequin and not actually wearing it. Any ideas of how to do this? This is what I've tried (a script inside the mannequin):

script.Parent:WaitForChild("Character"):WaitForChild("Humanoid"):AddAccessory(workspace.Jacket)