WebView - Define User-Agent on every request

Currently, I am setting the User-Agent of a request message as follows:

var rm = new HttpRequestMessage(HttpMethod.Post, new Uri("http://example.com"));       
rm.Headers.Add("User-Agent", "UserAgentString");
webView.NavigateWithHttpRequestMessage(rm);

Everything works as expected.

But, when I am navigating to an another page, by clicking a link on the site for example, the User-Agent resets to the default of the WebView.

Is there any way of setting the User-Agent permanently or changing it on every request made?

Thanks, George