add_filter('post_grid_media_source_list', 'post_grid_media_source_list_20200614', 10);

function post_grid_media_source_list_20200614( $sources){


    $sources['custom_source'] = __('Custom media source','post-grid');

    return $sources;

}

 

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How to add media source options

media_source_options_$id