WordPress function to get post id by slug and test it with an action hook to wp_head for tutorial - https://njengah.com/get-post-id-by-slug-in-wordpress/

wordpress_get_post_id_by_slug.php

<?php 

/**
 *  Function to get post ID By slug 
 */ 

function get_id_by_slug($page_slug, $slug_page_type = 'page') {
	$find_page = get_page_by_path($page_slug, OBJECT, $slug_page_type);
	if ($find_page) {
		return $find_page->ID;
	} else {
		return null;
	}
}

/**
 * Testing by Display to Header 
 */ 

add_action('wp_head' , 'test_function'); 

function test_function(){
	
	echo  get_id_by_slug('login-page','page');
	
}