If you are creating a Reminder along with SMS Invitations, click on Schedule Reminder Now and proceed from here.
If you wish to Schedule Remind Later, then follow the steps below.
How to Schedule Reminder?
Navigate to Survey>Builder>Launch.
In the Create Reminder message box, choose the Connection Type and add the a custom reminder message using the rich text editor. Then click Next.
Select if the Reminder should be sent to invitees who have Not responded or have only Partially Responded.
Next the Reminder has to be selected, i.e as One Time Invite or Recurring Invite.
For One-Time Invite For One-Time Reminder, configure the specified date and time, and then click Schedule.
For Recurring Invite If you are selecting Recurring invite, set the Frequency, Repeat, Schedule Trigger, Start Date,Time Slot(s) and Schedule Interval. Then click Schedule.