Preseeding d-i

Preseeding provides a way to set answers to questions asked during the
installation process, without having to manually enter the answers while the
installation is running. This makes it possible to fully automate most types of
installation and even offers some features not available during normal
installations. (https://wiki.debian.org/DebianInstaller/Preseed)

How to modify an existing CD image to preseed d-i see
https://wiki.debian.org/DebianInstaller/Preseed/EditIso

Using

$ bin/exec.sh DEBIAN_ISO_FILE DESTINATION_ISO_FILE