An invalid request URI was provided. The request URI must either be an absolute URI or BaseAddress must be set

You simply need to specify the full URL ( including the protocol) like this:

var response = await http.GetByteArrayAsync("http://www.nsfund.ir/news?p_....

An absolute URI follows the protocol://server/path?query#hash convention. As you did not specify a protocol, specifically http:// or https:// in your case, the URL is not absolute and thus cannot be resolved.