Schema Pro: Backorder availability option in product schema

gistfile1.txt

add_action( 'after_setup_theme', 'add_my_custom_meta_field' );
function add_my_custom_meta_field() {
    add_filter( 'wp_schema_pro_availability_options', 'my_extra_schema_field' );


function my_extra_schema_field( $fields ) {
    $fields['BackOrder '] = esc_html__( 'BackOrder ', 'wp-schema-pro' 
    );

    return $fields;
}
}