ACF

Add Custom Sub Page to and ACF Option Page

add_action('admin_menu', 'acf_option_custom_subpage', 99);
function acf_option_custom_subpage(){
    
    add_submenu_page(
        'my-parent-option-page',
        'My sub page',
        'My sub page',
        'manage_options',
        'my-sub-page',
        'my_sub_page_callback'
    );
    
}

function my_sub_page_callback(){
    
    // My sub page content...
    
}