Update an ExtendedProperties or Approval Comments of Sharepoint task item

I am trying to update a SharePoint task programmatically using CSOM. I am able to approve a task successfully but not sure how to update the comments.

The comments could be retrieved using ExtendedProperties as HashTable but I am unable to set the value for the same. How can I update the approval comments?

Below is the code to approve a task :

                using (ClientContext ctx = new ClientContext("http://abc/sites/TLS/low"))
            {
                Web web = ctx.Web;
                List oList = web.Lists.GetByTitle("Tasks");
                ListItem listitem = oList.GetItemById(784);
                ctx.Load(listitem);
                ctx.ExecuteQuery();
                Hashtable ht = GetExtendedPropertiesAsHashtable(listitem);
                listitem["Completed"] = true;
                listitem["PercentComplete"] = 1;
                listitem["Status"] = "Approved";
                listitem["WorkflowOutcome"] = "Approved";
                listitem["FormData"] = "Completed";
                //listitem["__ModerationComments"] = "Sdfs";
                //listitem["ows_FieldName_Comments"] = "Sdfs";
                //ht["ows_FieldName_Comments"] = "sdsds";
                listitem.Update();
                ctx.ExecuteQuery();
            }

Below is code to get the ExtendedProperties of a task item :

        public static Hashtable GetExtendedPropertiesAsHashtable(ListItem task)
    {
        if (task == null)
        {
            throw new ArgumentNullException();
        }
        Hashtable properties = new Hashtable();
        string extProperties = (string)task["ExtendedProperties"];
        if (!string.IsNullOrEmpty(extProperties))
        {
            var reader = new XmlTextReader(new StringReader("<Root " + extProperties + " />"))
            {
                WhitespaceHandling = WhitespaceHandling.Significant
            };
            reader.MoveToContent();
            if (!reader.HasAttributes)
            {
                return properties;
            }
            while (reader.MoveToNextAttribute())
            {
                string propName = reader.Name.Substring(4);
                properties[propName] = reader.Value;
            }
        }
        return properties;
    }

You could try this to update the approval comments:

listitem["ExtendedProperties"] = "ows_FieldName_Comments='comment' ";