How to at an application to "Open at Login" on nix darwin?

I try to add an application that was installed with homebrew (managed by nix darwin) to the "Open at Login" settings under "General -> Login Items & Extensions".

I tried to add a launchd.user.agents entry, but that didn't work. The app is only adeded to the "Allow in the Background" settings and does not start on login.

  launchd.user.agents = {
    sanesidebuttons = {
      serviceConfig = {
        Label = "com.thealpa.sanesidebuttons";
        RunAtLoad = true;
        Program = "/Applications/SaneSideButtons.app";
      };
    };
  };

Any ideas how to add an entry to the "Open at Login" settings with nix darwin? launchd.agents and launchd.daemons seems to be the wrong place as well.