What your code is doing is making html that will cause the browser to request the url and show the result, but that's not what you want, you want your code-behind to request the url and get the result. To do that you'll need to POST to the url programatically. Look at the "HttpClient" post code in the link below
C# – Several Ways To Perform Http GET and POST Requests | Yusuf Setiawan's Blog[
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Your dictionary of values will be things like
"TransactionID", Request.QueryString["TransId"],