Yea! Vit3D said it correctly!
On the modifier stack, apply a MeshSelect Modifier right on top of the stack.
Then at the Mesh Select Modifier set it to VERTEX mode.
Deselect all possible vertex and carefully select
those vertex going to be the pinned mesh.Name the selection set using the selection window on top.
Do not forget to hit ENTER after naming the selection.Now save the prop.
At iClone, import the prop.
Select the Prop!Then press SHift-Alt-S You will get to the Soft attributes hidden dialog.
At the Pinning Vertex Group window drop down, you will see the selection name you created.
Select it and
set it Active.Finally Pick the Target bone on the character. (Repeat until you get the RL_NeckLo bone.
Your prop should be now pinned.
Don't be too happy now! When the avatar walks you will have wild motions. Play with the Physics settings until you get it somewhat working fine!
iClone is pretty BAD for handling cloth simulation! Be warned!

Mike Aparicio-Reallusion Certified Innovative Content Developer