ASP.Net: An object reference is required for the nonstatic field, method, or property

I am assign value to hidden field as static but I want I assign this value as a dynamic I will try it but can’t success following my code which is below  .

ObjectParameter a ;
        a = new ObjectParameter("id", typeof(global::System.Int32));
        a.Value = DBNull.Value;

using (var db = new TestEntities())
        {

              int newdata=  db.TestBTBill(

                 Name=name,
                 a ……………………………………….. this is output parameter  
                 );
              HiddenField hfID;
              hfID.Value = a.ToString();

But that value can’t assign as dynamically .

My hidden field:

    <input runat="server" type="hidden" value="" id="hfID"/>
Here I get the value of the hidden field which are written on the client side.
   var CurrencyTransInfoId = ($("[id*=ContentPlaceHolder1_hfBTBillID]").val());

 please help me how I assign this value as a dynamically