Password-protect a custom rss feed

I’ve created a custom post type and would like to send those posts to FB Instant Articles and Apple News via a RSS feed. I’ve created a custom RSS feed and need to know what’s the best way to make that feed password-protected. The CPT is called limitedrun. The feed URL is domain.com/?feed=ltdrun and the RSS template is called rss-ltdrun.php

I know there are plugins that do this but it’s not what I’m looking for. They password protect the entire site in order to make the RSS feed password-protected. I need the opposite: RSS password-protected and the website open.

Thanks,

I would consider this not really a typical WP question – maybe you should ask over at http://unix.stackexchange.com – but maybe this could already help anyways:

If you have access to the server config and you know the IP address of FB Instant Articles and Apple News you could for example block everybody from accessing your feed – but allow FB Instant Articles and Apple News to access it – with something similar to this:

<Location /feed>
Order Deny,Allow
Deny from all
Allow from 1.1.1.1
</Location>

…as answered on unix.stackexchange.com (this will however NOT work in .htaccess)