How to add a custom tab to the Add Site screen

add_action( 'mainwp_add_site_top_navigation', 'mycustom_add_site_tab_navigation', 10, 2 );
function mycustom_add_site_tab_navigation( $active, $item_link ) {
    $tab     = 'custom-site';
    $classes = 'item' . ( $tab === $active ? ' active' : '' );
    $href    = $item_link
        ? ' href="' . esc_url( admin_url( 'admin.php?page=managesites&do=new&menu_active=' . $tab ) ) . '"'
        : '';

    printf(
        '<a class="%1$s" data-tab="%2$s"%3$s><i class="puzzle piece icon"></i>%4$s</a>',
        esc_attr( $classes ),
        esc_attr( $tab ),
        $href,
        esc_html__( 'Custom Site', 'your-text-domain' )
    );
}

add_action( 'mainwp_add_site_tabs_content', 'mycustom_add_site_tab_content' );
function mycustom_add_site_tab_content() {
    $active  = isset( $_GET['menu_active'] )
        ? sanitize_key( wp_unslash( $_GET['menu_active'] ) )
        : 'single-site';
    $classes = 'ui bottom attached tab padded segment' . ( 'custom-site' === $active ? ' active' : '' );

    printf(
        '<div class="%1$s" data-tab="custom-site"><h2 class="ui header">%2$s</h2><p>%3$s</p></div>',
        esc_attr( $classes ),
        esc_html__( 'Custom Site', 'your-text-domain' ),
        esc_html__( 'Add your custom fields or instructions here.', 'your-text-domain' )
    );
}