This scenario isn't supported for security reasons, because it would otherwise allow malicious code to access the app. The app and Webview run in separate script contexts, so you cannot share DOM elements and such. Instead, you might be able to place more
of the app structure that you need to share inside the Webview itself, so that you can achieve the necessary interaction.
Kraig
Author,
Programming Windows Store Apps with HTML, CSS, and JavaScript, Second Edition,
a free ebook from Microsoft Press.