add_action( 'admin_menu', function() {
	add_menu_page(
		esc_html__( 'Some Page', 'textdomain' ),
		esc_html__( 'Some Page', 'textdomain' ),
		'edit_posts',
		'some-page',
		'prefix_render_admin_menu_page'
	);
});

add_action( 'cmb2_admin_init', function() {
	$cmb = new_cmb2_box( array(
		'id'           => 'some_sub_page_metabox',
		'title'        => __( 'Some Sub-Page', 'textdomain' ),
		'object_types' => array( 'options-page' ),
		'option_key'   => 'some_sub_page',
		'parent_slug'  => 'some-page',
	) );
});

function prefix_render_admin_menu_page() {
	echo '<h1>Some Page</h1>';
}