As far as I know you can’t do that in the editor, so you either need to buzz over to Blender, or get something like this:
EDIT: I just saw another workaround for those not willing to part with time or cash:
“subtract a vector from the world position offset in the material. Then you can set it per material”
EDIT2: Incidentally, if you used the material method, then you could vary it with the PerInstanceRandom node…