How to Add custom menu to Bottom App Bar?

After so much research I finally found the solution to the problem. Just by changing namespace of 'showAsAction' from 'android' to 'app' make things work.

<item
    android:id="@+id/navigation_explore"
    android:icon="@drawable/ic_search"
    android:title="Explore"
    app:showAsAction="always" />