I have a mymail.php script which sends a email (I managed to do it using require_once( $_SERVER['DOCUMENT_ROOT'] . '/wp-includes/class-phpmailer.php' );.) and outputs a plain text string (Such as “Email is sent successfully.”)

I want to use the functions defined in WP in this my script. The particular function I want to call is get_option() in order to retrieve the email user of the site owner. Which file to import for the particular get_option() function and which to import the entire WP core (like the context available for theme’s .php files?