How to use ViewState in AJAXMethod or WebMethod in ASP.Net

I want to store the contents of Comment in ViewState when user is not login. After Login i want to redirect him on that page where he has filled comments and i want show the contents in comments filled by him.For this i am storing the comments'content in ViewState using AjaxMethod and JavaScript but it is not working for me. How i will do it,Please suggest me. My code is given below: 

[Ajax.AjaxMethod(Ajax.HttpSessionStateRequirement.ReadWrite)]
    public string InsertComments(string session,string comment)
    {
        if (session != "0")
        {
            try
            {
                UserDTO objInsert = new UserDTO();
                UserBAL objInsertComments = new UserBAL();
                objInsert.EmailId = Session["usr"].ToString();
                objInsert.URL = strURL;
                objInsert.Comments = comment;
                string c = objInsertComments.Insert_UsersComment(objInsert);
                int i = Int32.Parse(c);
                return "success";
            }
            catch (Exception ex)
            {
                return "Sorry! Your request can not be updated at this moment. \n\n Error Details:\n" + ex.Message + "\n\n We suggest you to try again in few moments. If this happens very often, please do contact admin.";
            }
        }
        else
        {
           
            ViewState["GIComment"] =comment;          
            return "Error.";
        }
    }

On Page Load i am writing following code: 

 if (ViewState["GIComment"] != null)
            txtComment.Text = ViewState["GIComment"].ToString();
        else
            txtComment.Text ="";