Script runner copy value between stories

[Marcela Junyent]

I have a script to copy value from the epic to story, a need to copy from stories using as "parent story" an issuekey from a custom field "Original Story":

The script that I'd been trying to adapt is:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.project.version.Version

def customFieldManager = ComponentAccessor.getCustomFieldManager()
def epicLinkCf = customFieldManager.getCustomFieldObjectByName("Epic Link")
def epicIssue = issue.getCustomFieldValue(epicLinkCf) as Issue
def targetRelCf = customFieldManager.getCustomFieldObjectByName("FieldB")

if (epicIssue) {
return (epicIssue.getCustomFieldValue(targetRelCf))
}

But I couldn't do it. Anyone knows how  to do it?