I used a WebView to browse a url, but it needs flash.
I tried
WebViewSettings ws = webView.Settings;
ws.IsJavaScriptEnabled = true;
Flash is not supported in UWP WebView.
In apps compiled for Windows 10, WebView uses the Microsoft Edge rendering engine to display HTML content. In apps compiled for Windows 8 or Windows 8.1, WebView uses Internet Explorer 11 in document mode. It does not support any Microsoft ActiveX controls or plugins like Microsoft Silverlight or Portable Document Format (PDF) files.
Source: Microsoft docs.
See also this MSDN blog post, as this confirms the statement.
WebView doesn’t do Flash. Or Silverlight. Or PDF. Or any other ActiveX control or plugin.