You can message a transport provider over the website before you've booked. Just log on to the website, open the job you've listed and message the provider from there.
When you click on your current open job you can click the "ask question" option which is just underneath the "view details" option. You can ask them anything you want to know about their service and let them know any details you couldn't enter when placing the job on the site.
Once you've booked you'll get their full contact details to discuss the job.