New Customer
First of all, you need to enable Remote Completion. You do this under settings->configuration->scheduling
Next, when adding a new customer, the Email or remote completion will be on Step 4 of the customer creation process. If you don't see that button, you either don't have remote completion enabled or you haven't entered a customer email in Step 1 of customer creation.
Existing Customer
Go to the customer->actions->resend email->remote completion