One component of the telehealth feature is the ability to chat with your patients via the instant messenger tool!
When you're in the video call, look for the little "chat" icon in the upper-right of the screen:
Pressing that will open up the chat, and you can type back and forth with your patient! To close it, press the little arrow in the upper-right:
This will work on any device—your computer, tablet, or mobile phone!