you do this by creating a butane config yaml and converting it to an ignite json using butane.

dnf install -y butane

Create a yaml file with your config, mid the indentation. In this example only a group and a user with a specific uid is created (an exhaustive list of the options for butane config can be found here) i name this file version1.bu

version: 1.6.0
passwd:
  groups:
    - name: beheerders
      gid: 700
  users:
    - name: abel
      uid: 700
      primary_group: users
      groups:
        - beheerders
      ssh_authorized_keys:
        - ssh-ed25519 AAAAC3................P abel@nergens

Then with butane transform it ti the ignite json: butane –pretty –strict version1.bu > version1.ign

the resulting json file version1.ign looks like:

{
  "ignition": {
    "version": "3.5.0"
  },
  "passwd": {
    "groups": [
      {
        "gid": 700,
        "name": "beheerders"
      }
    ],
    "users": [
      {
        "groups": [
          "beheerders"
        ],
        "name": "abel",
        "primaryGroup": "users",
        "sshAuthorizedKeys": [
          "ssh-ed25519 AAAAC3N................P abel@nergens"
        ],
        "uid": 700
      }
    ]
  }
}

https://www.flatcar.org/docs/latest/provisioning/config-transpiler/configuration/

https://docs.fedoraproject.org/en-US/fedora-coreos/bare-metal/

https://www.redhat.com/en/blog/red-hat-enterprise-linux-coreos-customization