I've created the script below to transition related tickets. I tested it using Script Runner and by adding an issue as a string in .getIssueObject().
def issue = constantsManager.getIssueManager().getIssueObject('AM-1234')
How can I get/pass the current issue that is transitioning into the scope of the Post Function? New to Groovy/JAVA and JIRA. 
// Transition script to transition an inWardLinked issue status when related link is approved
import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.IssueManager
import com.atlassian.jira.issue.link.IssueLink
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.workflow.WorkflowTransitionUtil
import com.atlassian.jira.workflow.WorkflowTransitionUtilImpl
import com.atlassian.jira.util.JiraUtils
def constantsManager = ComponentManager.getInstance()
WorkflowTransitionUtil workflowTransitionUtil = ( WorkflowTransitionUtil)JiraUtils.loadComponent( WorkflowTransitionUtilImpl.class )
def issue = constantsManager.getIssueManager().getIssueObject('AM-1234')
def issueLinkManager = constantsManager.getIssueLinkManager()
def relatedLinks = issueLinkManager.getInwardLinks(issue.id)
relatedLinks.each {issueLink ->
def linkedIssue = issueLink.getSourceObject()
// If related linktype is not in ABTEST project and it's status is "in design" transition status to approved
if (issueLink.issueLinkType.name == "Related" && linkedIssue.getProjectObject().getKey() != "ABTEST" && linkedIssue.getStatusObject().getName() == "In Design"){
workflowTransitionUtil.setIssue(linkedIssue)
workflowTransitionUtil.setAction(421) // design approved - see text view of workflow to get transition ids
workflowTransitionUtil.progress()
}}