Replaces a settings subpage link with a link to the customizer or external URL

admin-menu-customizer-link.php

<?php

add_action( 'admin_init', 'cameronjonesweb_add_settings_link' );
add_filter( 'clean_url', 'cameronjonesweb_admin_menu_customizer_link', 10, 3 );

function cameronjonesweb_add_settings_link() {
	add_options_page(
		'My Settings Page Title',
		'My Settings Page Menu Title',
		'manage_options',
		'my-settings-page',
		'__return_false'
	);
}

function cameronjonesweb_admin_menu_customizer_link( $good_protocol_url, $original_url, $_context ) {
	if ( 'options-general.php?page=my-settings-page' === $good_protocol_url ) {
		$good_protocol_url = admin_url( 'customize.php?autofocus[panel]=my-panel' );
	}
	return $good_protocol_url;
}

admin-menu-external-link.php

<?php

add_action( 'admin_init', 'cameronjonesweb_add_settings_link' );
add_filter( 'clean_url', 'cameronjonesweb_admin_menu_external_link', 10, 3 );

function cameronjonesweb_add_settings_link() {
	add_options_page(
		'My Settings Page Title',
		'My Settings Page Menu Title',
		'manage_options',
		'my-settings-page',
		'__return_false'
	);
}

function cameronjonesweb_admin_menu_external_link( $good_protocol_url, $original_url, $_context ) {
	if ( 'options-general.php?page=my-settings-page' === $good_protocol_url ) {
		$good_protocol_url = 'https://cameronjonesweb.com.au';
	}
	return $good_protocol_url;
}

admin-menu-override-global.php

<?php
add_action( 'admin_menu', 'cameronjonesweb_add_menu_item' );

function cameronjonesweb_add_menu_item() {
	global $submenu;
	$url = 'https://cameronjonesweb.com.au';
	$submenu['options-general.php'][] = array( 'My custom menu link', 'manage_options', $url );
}