Yes, you are required to enter your appointment on the thera-LINK calendar for online sessions. This way, your clients receive the email to join the session, and it shows up when they log in to join.
You will also receive a calendar invite when the appointment is scheduled. With any mail program, simply click ACCEPT or YES, and it will be added to the calendar associated with that mail application. For instance, if you use Gmail, just click YES and now that thera-LINK appointment will show up on your Gmail calendar. The Gmail calendar is probably synced on your iPhone or Android device, so you'll have that appointment listed on your device.
Here's what the Calendar invite looks like on Gmail:
Don't worry - there's no PHI included in the calendar invite. :)