To message your provider, login to your account and go to your "Messages" tab located under your profile picture. From there, click "Compose" and select your provider from the "Recipient" drop-down. If you have a document you would like to attach to the message you can do that here as well. Once you have entered a subject and message, remember to click the "Send" button.
*Note: The messaging feature is not meant for treatment purposes, but more for scheduling coordination and casual conversation.