To send messages, you need a number, which costs $3 USD per month. Before buying one, add funds to your account.
- In the top menu, go to Tools & Add-ons, select Communication, and then SMS/MMS Messaging.
- In New message, next to the From field, click Buy number.
- In Select New Number, choose the number and confirm.
If you don't have any funds, the window will let you know and you'll be able to add them from there.