Ubuntu 24.04 - Autoinstall and debconf-selections

I am trying to use autoinstall for the first time.
After first issues I was able to create a basic autoinstall.yaml and to install
a fresh system with that file.
Now I wanted to modify the autoinstall.yaml to install KDE next to Gnome.
In an interactive shell dpkg-reconfigure asks which display manager should be used.
To automate the answer I can use the keyword "debconf-selections", but whatever I try the installer tells me this error:
Malformed autoinstall in "debconf-selections" section

First the whole install file:

autoinstall:
  version: 1
  interactive-sections:
    - network
    - locale
    - keyboard
  identity:
    realname: 'Testuser'
    username: testuser
    hostname: 'my-special-pc'
    password: '$6$SomeHash'
  storage:
    layout:
      name: lvm
      sizing-policy: all
  codecs:
    install: true
  drivers:
    install: true
  oem:
    install: auto
  snaps:
    - name: brave
    - name: opera
  packages:
    - ansible
    - arandr
    - kubuntu-desktop
  debconf-selections:
    - d-i shared/default-x-display-manager select sddm
  timezone: geoip
  updates: all

Alternatives I tried and remember:

  debconf-selections:
    - sddm shared/default-x-display-manager select sddm

  debconf-selections:
    - shared/default-x-display-manager select sddm

What am I doing wrong?
P.S.: If there are other issues, I will deal with them later, but for now the autoinstall
works if I remove "debconf-selections".