Web Forms :: How To Use ViewState In AJAXMethod Or WebMethod

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, 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();

[Code].....

Error Details:
" + ex.Message + "

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 ="";