$ ./result/bin/otbr-agent --help
Usage: ./result/bin/otbr-agent [-I interfaceName] [-B backboneIfName] [-d DEBUG_LEVEL] [-v] [--auto-attach[=0/1]] RADIO_URL [RADIO_URL]
    --auto-attach defaults to 1
RadioURL:
    forkpty-arg[=argument string]  Command line arguments for subprocess, can be repeated.
    spinel+hdlc+uart://${PATH_TO_UART_DEVICE}?${Parameters} for real uart device
    spinel+hdlc+forkpty://${PATH_TO_UART_DEVICE}?${Parameters} for forking a pty subprocess.
Parameters:
    uart-parity[=even|odd]         Uart parity config, optional.
    uart-stop[=number-of-bits]     Uart stop bit, default is 1.
    uart-baudrate[=baudrate]       Uart baud rate, default is 115200.
    uart-flow-control              Enable flow control, disabled by default.
    uart-reset                     Reset connection after hard resetting RCP(USB CDC ACM).
    region[=region-code]          Set the radio's region code. The region code must be an
                                  ISO 3166 alpha-2 code.
    cca-threshold[=dbm]           Set the radio's CCA ED threshold in dBm measured at antenna connector.
    enable-coex[=1|0]             If not specified, RCP coex operates with its default configuration.
                                  Disable coex with 0, and enable it with other values.
    fem-lnagain[=dbm]             Set the Rx LNA gain in dBm of the external FEM.
    no-reset                      Do not send Spinel reset command to RCP on initialization.
    skip-rcp-compatibility-check  Skip checking RCP API version and capabilities during initialization.

{
  lib,
  stdenv,
  fetchFromGitHub,
  cmake,
  ninja,
  pkg-config,
  avahi,
  cjson,
  http-parser,
  systemd,
  cpputest,
}:

stdenv.mkDerivation rec {
  pname = "ot-br-posix";
  version = "20230710";

  src = fetchFromGitHub {
    owner = "openthread";
    repo = "ot-br-posix";
    rev = "thread-reference-${version}";
    hash = "sha256-q7SHmFLfCUYPB7WEUSyYL6hI8/tbkkJkDJ0FBJVwffw=";
    fetchSubmodules = true;
  };

  postPatch = ''
    rm -rf third_party/{cJSON,http-parser}
  '';

  nativeBuildInputs = [
    cmake
    ninja
    pkg-config
  ];

  cmakeFlags = [
    "-Dsystemdsystemunitdir=${placeholder "out"}/lib/systemd/system"
    "-DOTBR_SYSTEMD_UNIT_DIR=${placeholder "out"}/lib/systemd/system"
  ];

  buildInputs = [
    avahi
    http-parser
    cjson
    #systemd
  ];

  # greps /var/log/syslog in the build sandbox
  doCheck = false;

  nativeCheckInputs = [ cpputest ];

  meta = {
    description = "OpenThread Border Router, a Thread border router for POSIX-based platforms";
    homepage = "https://github.com/openthread/ot-br-posix";
    license = lib.licenses.bsd3;
    maintainers = with lib.maintainers; [ ];
    mainProgram = "ot-br-posix";
    platforms = lib.platforms.all;
  };
}

In reply to @hexa:lossy.network
also uses a vendored submodule for openthread

ExecStartPre=service avahi-daemon start