Here's what I have in my conf :
```
  systemd.services.i3lock = {
    description = "i3lock";
    before = [ "sleep.target" ];
    serviceConfig = {
      User = "matthieu";
      Type = "forking";
      Environment="DISPLAY=:0";
      ExecStart=''${pkgs.i3lock}/bin/i3lock'';
    };
    wantedBy = [ "sleep.target" ];
    enable = true;
  };
```