Static Message: For sending a static message just enter the Text Message as shown in below format and click Create.
Dynamic Message: For sending dynamic or custom message like including Name, date & time of appointment or any other customize message which include any of the fields associated with the contact. To add a field value in your message use the contact field name within [[...]]. Check below image for reference.
For an example, let's say you have following fields for a contact as shown in the image below.
Then you can create a customized notification message for appointment reminder like shown in below image. You can also click the link "Click here" to learn the format anhed know about the fields you need to use.
The output of above message would be like below.