The Add Message button is intended to be used when you need to relay 

Click Message.

Enter the message in the text field and verify the cell phone number to which you'd like to send the message.  

The client's cell phone number will populate automatically if one is listed.  Other phone numbers in the client's chart will also be listed here, as well as the option to manually enter a phone number that doesn't exist in the client's chart.

Because you aren't expecting a response, this will immediately be logged under History of Client Communications.