You can send a message to experts to ask clarifying questions before requesting a call by going to their profile, and clicking on the little envelope icon that is part of the "Schedule a call" button.
Once you request a call, both you and the expert will be able to send each other messages.
To send a message, go to your Calls page > Select the member/expert you'd like to send a message to > Click "Send a Message".
You can use the secure messaging service on Clarity to work out the details of your call request, fill in an expert before your call to save time during the call, or just convey any correspondence you feel necessary.
NOTE: In order to add separate lines in a message, simply click Shift+Enter or Ctrl+Enter on Windows/Linux.
ANOTHER NOTE: Experts can only send messages to you if you've messaged them first, or requested a call.