Finding the control id in the form ?

 
  Hello,
 
I have a web form application and in this application I also have a public class file located in the App_Code folder.  This class cannot be inherited (Class System.SerializationAttribute).  In this class, I want to locate one of the control id in the form and using that control to overide the value on the field, how do I do it?  Thanks
 
 
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