I don’t have a Core Service version, but here is one I wrote using VBScript and TOM which might get you started in the right direction
Set objAIs = CurrentWorkItem.ActivityInstance.ProcessInstance.ActivityInstances
Set objAI= objAIs (objAIs.Count - 1)
strLastMessage= objAI.FinishMessage
Set objAI= objAIs(1)
uriFirstUser= objAI.Performer.ID
Set objAI= Nothing
Set objAIs = Nothing
FinishActivity cstr(strLastMessage), "" , uriFirstUser
Just paste that in the script box of your automated activity, and you should be good to go
solved How to send the activity back to author using coreservice?