How to change a specific page to custom page?

I think you want a custom page template.

1. Create a custom page template.
2. In that page template paste your code.
3. Create a “Page” from wp_admin->Pages and select your template file as the “Template”

In your page template, you would have something like this:

/*
Template Name: My Custom HTML Page
*/
get_header();

get_footer();

Leave out the get_header and get_footer and you will have a page that loads in WordPress but without most of the theme structure.