How to create a custom WordPress Rest Api route. https://since1979.dev/snippet-005-simple-custom-rest-api-route/

snippet-004-register-rest-route.php

<?php
/**
 * mytheme_get_meta_value.
 *
 * Register a custom Wp Rest Api end-point,
 *
 * @see https://since1979.dev/snippet-005-simple-custom-rest-api-route/
 *
 * @uses register_rest_route() https://developer.wordpress.org/reference/functions/register_rest_route/
 * @uses array() https://www.php.net/manual/en/function.array.php
 */
function mytheme_get_meta_value()
{
    register_rest_route('mytheme/v1', '/meta/(?P<post>[\d]+)/(?P<key>[a-zA-Z0-9_-]+)', array(
        'methods' => WP_REST_Server::READABLE,
        'callback' => 'mytheme_handle_get_meta_value',
    ));
}

/**
 * Hook: rest_api_init.
 *
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 * @uses rest_api_init https://developer.wordpress.org/reference/hooks/rest_api_init/
 */
add_action('rest_api_init', 'mytheme_get_meta_value');
?>