prefix_register_custom_meta.php

//Register Meta with Rest API
add_action( 'rest_api_init', 'prefix_register_custom_meta' );
function prefix_register_custom_meta() {
    register_rest_field( 
        'post',
        '_prefix_url',
        array(
            'get_callback'    => 'prefix_get_custom_meta',
            'update_callback' => null,
            'schema'          => null,
        )
    );
    register_rest_field( 
        'post',
        '_prefix_email',
        array(
            'get_callback'    => 'prefix_get_custom_meta',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}