Yes, you can customize sign-up and sign-in by adding app-specific logic to the user sign-up and sign-in flows using AWS Lambda. For example, you can create AWS Lambda functions to identify fraud or perform additional validations on user data. You are able to trigger developer-provided Lambda functions at pre-registration, at post-confirmation, at pre-authentication, during authentication to customize the challenges, and at post-authentication. You can also use Lambda functions to customize messages sent as part of email or phone number verification and multi-factor authentication.