How to automate the installation from ISO?

I try to perform unattended installations of Ubuntu 20,04.05 using the "ubuntu-20.04.5-live-server-amd64.iso" iso.

Due to the absence of the `boot_command`, there seem to be no way to influence how the installer is started from the iso.
Adding `userdata` with "#cloud-config/autoinstall" has no effect on the Subiquityinstaller, if there is no possibility to tell Subiquity that it should be used and where it can be accesses from..

Oddly, the `boot_command` is mentioned in the description of `http_directory` and `http_content`, but it doesn't seem it is implemented for the CloudStack provider.

What am I missing here? How do we make the CloudStack provider use userdata for Subiquity, preseed.cfg for Debootstrap or kickstart. ks for Kickstart?