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add_filter( 'nav_menu_link_attributes', 'add_custom_atts_3032', 10, 3 );
function add_custom_atts_3032( $atts, $item, $args )
{
    // The ID of the target menu item
    $menu_target = 3032;

    // inspect $item
    if ($item->ID == $menu_target) {
        $atts['data-toggle'] = 'modal';
    }
    return $atts;
}