First install the NixOS module for the server components of smos. Then activate the api server as follows:

{
  services.smos.production = {
    enable = true;
    api-server = {
      enable = true;
      log-level = "Warn";
      hosts = [ "api.smos.example.com" ];
      port = 8402;
      local-backup = {
        enable = true;
      };
    };
  };
}

See also the reference documentation for this nixos module.