/**
 * Make the custom popup content available in the REST API response.
 */
function expose_popup_content_in_rest_api() {

    register_rest_field(
        'te_announcements',
        'popup_content',
        array(
            'get_callback' => function( $object, $field_name, $request ) {
                return get_post_meta( $object['id'], 'announcement_popup_content', true );
            },
            'schema' => null,
        )
    );

}
add_action( 'rest_api_init', 'expose_popup_content_in_rest_api' );