Develop WordPress site. Have non-WP live site. Make WP live site when ready

We currently have a live site that is not WordPress. We plan to develop a new WordPress site to replace the live site.

I know I can install WordPress with a live non-WordPress site, but how do I make WordPress the live site when it’s ready?

In other words when I install WordPress the site may be mydomain.com/wpsite.

When the WordPress site is live I want the URL to be mydomain.com I don’t want the /wpsite folder name in the URL

What is the best way to do this ?

Is it as simple as just putting this code in my index.php file in my website root folder ?
define(‘WP_USE_THEMES’, true);

/** Loads the WordPress Environment and Template */
require( dirname( __FILE__ ) . ‘/wp-blog-header.php’ );

I thought about htaccess but would that mess WP up ?

Your help is much appreciated.

Thank You!