I’m customizing Calamares. In the umount.conf module there is a line
exec: "/usr/lib/calamares/postinstall.sh"
So at the end of the installation process the postinstall.sh script is launched from the umount.conf module and do modifications (in my case launching the paxrat utility). But in my case it is NOT launched. I even added debug information
Perhaps whatever distro you’re trying to install (you’ve never stated which one), their forum might have more specific info to assist you. Especially if it’s a distro-specific customization.