Once you have Adafruit IO+ up and running, navigate to the Power-Ups tab and find the SMS option. Click "Get Started". 

Click "Set up your Verified Phone Number" and follow the prompts to enter the number you want to use as your texting phone.

This will only work with one verified phone number per Adafruit IO+ account so be sure you're using the phone you want to use to text the sign.

Once the number is set up, click "Activate Phone Number" and follow the prompts to activate.

The activation window will give you a phone number to send your text messages to. Save this in your phone as a contact called "Adafruit IO" so you can find it later.