How to get additional information via bing search api?

It sounds pretty unclear, but probably, by "information under the link", you simply mean content the found URL points to. You simply need to download it and present somehow, using… whatever you have. This is done using the class System.Net.HttpWebReques or System.Net.Http.HttpClient:
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest%28v=vs.110%29.aspx[^],
http://msdn.microsoft.com/en-us/library/system.net.http.httpclient%28v=vs.118%29.aspx[^].

See also my past answers:
get specific data from web page[^],
Performing a some kind of Web Request and getting result[^].

—SA