add_action('rest_api_init', 'register_custom_endpoints');

function register_custom_endpoints(){
    register_rest_route($base, $endpoint, [
        'methods' => 'POST',
        'callback' => function($request){
            // Return JSON data
        },
        'permission_callback' => function($request){
            // This always returns false
            return is_user_logged_in();
        },
    ]);
}

{code: "rest_user_invalid_id", message: "Invalid user ID.", data: {status: 404}}