How to add WP Admin button to the main Navigation bar

add_filter( 'mainwp_main_menu', 'mycustom_mainwp_main_menu', 10, 1 );
function mycustom_mainwp_main_menu( $left_menu ) {
    $index = 10; // Position in the menu
    $sub_menu_after = array_splice( $left_menu['leftbar'], $index );

    $addition_item = array();

    $addition_item[] = 'WP Admin';           // Menu item title
    $addition_item[] = '';                   // Parent key
    $addition_item[] = 'index.php';          // URL to WP Admin dashboard
    $addition_item[] = 'custom-wp-admin-item'; // HTML ID attribute
    $addition_item[] = '';                   // Icon (optional)

    $left_menu['leftbar'][] = $addition_item;
    $left_menu['leftbar'] = array_merge( $left_menu['leftbar'], $sub_menu_after );

    return $left_menu;
}

#custom-wp-admin-item {
    padding-left: 10px;
    padding-right: 10px;
    position: absolute;
    bottom: 0;
}