You can do this by creating a recurring campaign based on an attendance report that only captures patients with just 1 appointment. For example, a campaign that targets patients with only 1 total appointment, three days before their appointment, repeated every day, will automatically send communications to your new patients three days before their first appointment. It will not target your existing patients, because they will have more than 1 appointment. It will also not target the same patient twice, because the parameters specify they must have exactly three days to their next appointment.
To create such a campaign:
Click Reports > Appointment.
Click New Report at the top-right of the page.
In the Report Type field, select Attendance.
In the Length fields, select "Equals" and choose a number of days, weeks or months before the next booked/confirmed appointment.
In the Appointments field, choose 1 total appointment.
Click Run. This will create a report that only shows patients with 1 booked appointment X number of days/weeks/months away.
Click Save and give the report a title and description you will recognise.
Click Set as Campaign.
In the Save Campaign pop-up, choose the SMS or email template you wish to use.
Click Repeat Campaign and specify how often you want the campaign to repeat.
RESULT: You have now created a campaign that will run according to your specifications, targeting only new patients as recipients.