First of all, I feel the approach need to reviewed and consider about better approach to get data from client rather than getting it via querystring directly inputted by the client. For example, you get this via a textbox and then encoding the value correctly and send via querystring or other options as per your requirement.
However, if your requirement needs this to be done really with bare querystring manipulation, you can try setting following in the web.config file under
system.web
tags.
<globalization fileEncoding="iso-8859-1" requestEncoding="iso-8859-1"responseEncoding="iso-8859-1"/>
Reference:
How to get Correct QueryString Value from URL having UNICODE value in asp.net? - Stack Overflow[
^]
Hope, it helps :)