You can set a default mobile message template that will auto-populate when you create a mobile message. This is helpful if you want to prefix your message with your organization or candidate's name, or add something like 'stop2quit' at the end of the message.
To set this up, navigate to the Settings tab of your group manage page. From there, scroll down to the 'Mobile Message Template' section and put in your preferred text, keeping an eye on the character count. Once you're done, save the page. From now on, every new mobile message you make, whether it's a mass mobile message or a mobile message in a ladder, will be automatically populated with the default text. The default text can be edited or removed.