<script>
            function pageLoad() {
                var mediaPlayer = $find('<%=RadMediaPlayer1.ClientID%>');
                var fullScreenButton = document.querySelector("#<%=RadMediaPlayer1.ClientID%>_Toolbar_FSButton");

                // Attach click event to the fullscreen button
                $telerik.$(fullScreenButton).on("click", function () {

                    var isFullScreen = !!(
                        document.fullscreenElement ||
                        document.mozFullScreenElement ||
                        document.webkitFullscreenElement ||
                        document.msFullscreenElement
                    );

                    // Check if we are in fullscreen mode
                    if (!isFullScreen) {
                        // If it is full screen mode, do something, e.g. Hide the playlist
                        mediaPlayer.playlist.set_visible(false);

                    } else {
                        // otherwise show the playlist
                        mediaPlayer.playlist.set_visible(true);

                    } 
                });
            }

        </script>
        <telerik:RadMediaPlayer ID="RadMediaPlayer1" runat="server" Width="600px" Height="400px" PlaylistPosition="Right" AutoPlay="true">
            <Playlist>
                <telerik:MediaPlayerVideoFile Path="video1.mp4" Title="Video 1" />
                <telerik:MediaPlayerVideoFile Path="video2.mp4" Title="Video 2" />
            </Playlist>
        </telerik:RadMediaPlayer>