msPlayToSource property

<video id="videoplayer" src="http://www.contoso.com/clip.mp4" controls autoplay />
<script type="text/javascript">
    // Step 1: Obtain PlayToManager object for app’s current view.
    var ptm = Windows.Media.PlayTo.PlayToManager.getForCurrentView();

    // Step 2: Register for the sourcerequested event (user swipes Devices charm).
    ptm.addEventListener("sourcerequested", function(e) {

        // Step 3: Specify the media to be streamed (to filter devices).
        e.sourceRequest.setSource(document.getElementById("videoplayer").msPlayToSource);

        // The media will then be streamed to the device chosen by the user in the UI.

    });