Response.Write just ouputs a literal string to the Response Stream going to the client.
If you need this value to come back through the Request Stream then you'll need to add it something like this into your html and within your form so that you can pull it from the form data on the server later.
<input id="YourId" type="hidden" value="Your value goes here" />
Then in your code behind you can see this value like this
string yourValue = YourId.Value;
yourValue will equal "Your value goes here"...