You are able to add the option for clients to request a text message reminder while they're booking an appointment with you on the Client Scheduler at any time. To do this, you'll need to navigate to Settings > Client Information and locate the 'Clients can request text message' field -
From here you'll need to click on the blue Edit button on the upper right, check the box next to 'Clients can request text message', and then click on the green Save button.
This will cause the 'Display as checked by default for clients' box to appear directly under the 'Clients can request text message' field -
If you check the box next to this field, the box for requesting text message reminders will default to being checked on the Client Scheduler.
Now, on the Your Information panel of your scheduler, you'll see the option for clients to select that they'd like to receive a text message reminder for their appointment -
This field will only show if you are including the Cell Phone field from clients as they book their appointment. So if you are not asking for clients' cell phone numbers, then even if you have the 'Clients can request text message' box checked this prompt will not appear on your scheduler.