How can I set the secure flag on an ASP.NET session cookie?

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C#

ASP.NET

security

session

cookie

need to set the secure flag for session cookies.

I have tried like below but session will null, displays Session Expired. Please login again.

<httpCookies   requireSSL="true" />

i'm setting secure flag for cookie as like above code in <system.web>

cs page code:

protected void Page_Load(object sender, EventArgs e)
   {
       if (Request.ServerVariables["HTTP_REFERER"] != null)
       {
           if (Request.Form["mwa"] != null && Request.Form["mwp"] != null)
           {
               String EmployeeNo = Request.Form["mwa"];
               String ApplicationID = Request.Form["mwp"];
               TripleDESSecureAlgorithm.CipherCode objTripleDes = new TripleDESSecureAlgorithm.CipherCode();
               String strEmpNumbDec = objTripleDes.Decrypt(EmployeeNo, ApplicationID);
               if (strEmpNumbDec != "")
               {
                   Session["EMPID"] = strEmpNumbDec;
               }
           }
           if (HttpContext.Current.Session["EMPID"] == null)
           {
               Response.Write("<center>Session Expired. Please login again.</center>");
               Response.End();
           }

and

<authentication mode="Forms">
    <forms loginUrl="~/Account/Login.aspx" timeout="2880"  />
  </authentication>

in <system.web>

Please help me in this case why current session is null.? or any other way to implement this ?

What I have tried:

I have tried like below but session will null, displays Session Expired. Please login again.

<httpCookies   requireSSL="true" />