Only allow one URL to access a website. Block all ofters in .htaccess

Question

I have a site (example.com) and on that site I have an iframe. I want that frame to bring in a site (site2.com) but not allow anyone else from directly accessing that site (site2.com) directly.

This .htacces code works on (example.com) but is only bring in the homepage of (site2.com)

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP_REFERER} !^https://example.com/$
    RewriteRule index.php - [F,NC]
</IfModule>