You can try to put
<system.web>
<identity impersonate="false" />
In web.config. This should send the end user credentials through the database. If this is not what you want, try something like
ImpersonableWebRequest request = WebRequest.Create(url);
CredentialCache creds = new CredentialCache();
NetworkCredential networkCredential = new NetworkCredential("bob", "130B", "domain");
creds.Add(new Uri(url), authType, networkCredential);
request.Credentials = creds;
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