How to send YouTube videos from your Android app to a Chromecast device: https://medium.com/@soffritti.pierfrancesco/how-to-send-youtube-videos-from-your-android-app-to-a-chromecast-device-541d59ea1260

layout.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >
  
    <android.support.v7.app.MediaRouteButton
         android:id="@+id/media_route_button"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content" />
  
</LinearLayout>