/**
 * Paste this in your active theme's functions.php file.
 * @param \GeminiLabs\SiteReviews\Request $request
 * @return void
 */
add_action('site-reviews/review/request', function ($request) {
    // This assumes that "surcharge" is the field name of the custom field.
    if ('Yes' === $request->get('surcharge')) {
        $request->set('assigned_posts', '13'); // Site Reviews category ID
    }
    if ('No' === $request->get('surcharge')) {
        $request->set('assigned_posts', '14'); // Site Reviews category ID
    }
});