Restrict Access to REST API (just like generatepress.com)

Hi there,

I have a somewhat peculiar support question. Would like to apologise in advance if this isn't appropriate.

I'm looking to restrict access to my site's REST API. I happened to chance upon generatepress.com/wp-json and I'm shown an error message, saying that '... only authenticated users can access the REST API.'

This is exactly what I need to implement for my site. Could you share the way you guys did it?

Ed