You have the option to include a personalized signature at the end of your messages. This signature can be a text of your choice, such as Thanks, Michael or Text STOP to opt-out.
It is important to note that custom signatures are associated with the user, not the phone number. If you use multiple lines, your signature will be the same for all of them.
When you send a message to one of your contacts you have the option to toggle your custom signature on/off for that SMS.
To turn your signature on/off click on the Signature icon above the text box.