How to update the comment of an issue in Jira

I am trying to concatenate some text to a comment entered by the user. How can I do that? Here is my code below. I need to access the comment input by the user and then add new text to it. How can I access this? I am using Jira ScriptRunner custom postfunction to be executed when the user clicks on a transition.

import com.atlassian.jira.issue.comments.Comment
import com.atlassian.jira.workflow.JiraWorkflow
import com.atlassian.jira.workflow.WorkflowManager
import org.apache.log4j.Logger
import com.atlassian.jira.component.ComponentAccessor


def log = Logger.getLogger("atlassian-jira.log")
log.warn("This is the last action ")

WorkflowManager workflowManager = ComponentAccessor.getWorkflowManager();
JiraWorkflow workflow = workflowManager.getWorkflow(issue);
List <Object> actions = workflow.getLinkedStep(issue.getStatus()).getActions(); 

def wfd = workflow.getDescriptor()
def actionName = wfd.getAction(transientVars["actionId"] as int).getName(); 

log.warn("This is the last action "+actionName)
def comment= "+++ added via workflow action "+"\""+actionName+"\"+++"

Here is the code transientVars is the way to go.

import com.atlassian.jira.issue.comments.Comment
import com.atlassian.jira.workflow.JiraWorkflow
import com.atlassian.jira.workflow.WorkflowManager
import org.apache.log4j.Logger
import com.atlassian.jira.component.ComponentAccessor


def log = Logger.getLogger("atlassian-jira.log")
log.warn("This is the last action ")

WorkflowManager workflowManager = ComponentAccessor.getWorkflowManager();
JiraWorkflow workflow = workflowManager.getWorkflow(issue);

def wfd = workflow.getDescriptor()
def actionName = wfd.getAction(transientVars["actionId"] as int).getName(); 

log.warn("This is the last action "+actionName)
def comment= "+++ added via workflow action "+"\""+actionName+"\"+++"
String content = transientVars["comment"] +"\n"+comment  as String
log.warn("CONTENT"+ content)
transientVars["comment"]= content