Specifying the name of the URL parameters required by Calendly is currently not possible with the method you described. One option would be to use Keap’s built in Thank You page as the Thank You page with a ‘Click Here to Confirm a Meeting Time’ button that passes the desired URL parameters through to Calendly upon click.
Please review the following response for another Keap user where I showed them how to do this for ScheduleOnce: