User Agent gets overwritten so manually set to IE 10.0

After creating the HttpWebRequest as 

--- C# code ---

var request = HttpWebRequest.Create(destinationUri) as HttpWebRequest;
request.Headers["User-Agent"] = "Mozilla/5.0 (compatible; MSIE 10.0; Windows Phone 8.0; Trident/6.0; IEMobile/10.0; ARM; Touch"

--- End Code --

The User-Agent doesn't get populated as shown. (I checked using Wireshark and Fiddler).

I've put the hardcoded IE 10.0 Mobile user agent to resolve this issue. If there's a better way to set the User Agent of the HttpWebRequest it'll be nice.