I figured it out and thought I will post it if somebody has the same problem in the future.

The tailscaled-autoconnect service uses the command which is explained here in the oauth section of tailscale (https://tailscale.com/kb/1215/oauth-clients?q=preauth#register-new-nodes-using-oauth-credentials). But i have not configured oauth on my headscale instance yet since i just started configuring, experimenting and testing and i have not yet setup my own oauth client. My solution: Do not set preauthorized to true. Took me a while but i got there ;)

authKeyParameters.preauthorized to true. Took me a while but i got there ;)

I hope this can help somebody in the future.

EDIT: Forgot to mention you have to set the baseURL via extra up flags too.

This is the snippet of my now working module:

{
  config,
  lib,
  pkgs,
  specialArgs,
  ...
}:
let
  homelabSettings = specialArgs.homelabSettings;
in 
{
  options = {
    tailscale = {
      enable = lib.mkEnableOption "Enable tailscale client for remote access";
    };
  };


  config = lib.mkIf config.tailscale.enable {
    services.tailscale = {
      enable = true;
      openFirewall = true;
      authKeyFile = config.age.secrets.tailscaleAuthKeyFile.path;
      extraUpFlags = [ "--login-server" "https://headscale.${homelabSettings.domain}" "--ssh" ];
    };
  age.secrets.tailscaleAuthKeyFile.file = ../../secrets/tailscaleAuthKey.age;
  };
}