add_action( 'admin_menu', 'my_custom_menu_page' );
function my_custom_menu_page() {
add_menu_page(
__( 'Custom Menu Title', 'textdomain' ),
'Custom Menu',
'manage_options',
'custompage',
'my_custom_menu_page_content',
'dashicons-admin-generic',
);
}

add_action( 'admin_menu', 'my_custom_submenu_page' );
function my_custom_submenu_page() {
add_submenu_page(
'custompage',
__( 'Custom Submenu Title', 'textdomain' ),
'Custom Submenu',
'manage_options',
'customsubmenu',
'my_custom_submenu_page_content'
);
}