You add it as a device handler/driver, as @kahn-hubitat notes, and then you create devices for each phone number or email you want to send messages to. If you put in a phone number you have to use a number that your ISP allows an email to generate a text. For me on Verizon it's xxx-xxx-xxxx@vtext.com.
You then select those devices you created (essentially choosing what email addresses or phone numbers you want to send messages to) in your notification apps.