In this case, I believe we’re dealing with an installer variable that cannot be configured directly with installer flags. Take a look at these docs: Installing Foreman Server with Katello 4.15 plugin on Enterprise Linux
The file that you’d need to edit is /etc/foreman-installer/custom-hiera.yaml, which the docs appendix I linked has some info about.
There are essentially only two ways to describe the Foreman configuration to the installer. One is via the normal flags passed into foreman-installer, and the other is with custom hiera.