Flutter – How can I add the "system" message into my prompt?

I’ve been trying to integrate gpt-3.5-turbo in my Flutter app while maintaining the chat history. I used FlutterFlow to generate the boilerplate code and then downloaded the code to further edit it. I have successfully integrated the model while maintaining the chat history but I am unable to figure out how to add the "system" message into the prompt.

Here’s the API call code:

class OpenAIChatGPTGroup {
  static String baseUrl = 'https://api.openai.com/v1';
  static Map<String, String> headers = {
    'Content-Type': 'application/json',
  };
  static SendFullPromptCall sendFullPromptCall = SendFullPromptCall();
}

class SendFullPromptCall {
  Future<ApiCallResponse> call({
    String? apiKey = 'sk-xxxxxxxxxx',
    dynamic? promptJson,
  }) {
    final prompt = _serializeJson(promptJson);
    final body = '''
{
  "messages": ${prompt},
  "temperature": 0.8,
  "model": "gpt-3.5-turbo"
}''';
    return ApiManager.instance.makeApiCall(
      callName: 'Send Full Prompt',
      apiUrl: '${OpenAIChatGPTGroup.baseUrl}/chat/completions',
      callType: ApiCallType.POST,
      headers: {
        ...OpenAIChatGPTGroup.headers,
        'Authorization':
            'Bearer sk-xxxxxxxxxx',
      },
      params: {},
      body: body,
      bodyType: BodyType.JSON,
      returnBody: true,
      encodeBodyUtf8: true,
      decodeUtf8: true,
      cache: false,
    );
  }

  dynamic createdTimestamp(dynamic response) => getJsonField(
        response,
        r'''$.created''',
      );
  dynamic role(dynamic response) => getJsonField(
        response,
        r'''$.choices[:].message.role''',
      );
  dynamic content(dynamic response) => getJsonField(
        response,
        r'''$.choices[:].message.content''',
      );
}

Here’s _serializeJson() function:

String _serializeJson(dynamic jsonVar, [bool isList = false]) {
  jsonVar ??= (isList ? [] : {});
  try {
    return json.encode(jsonVar);
  } catch (_) {
    return isList ? '[]' : '{}';
  }
}

Here’s the code in the onPressed() function of the submit button:

setState(() {
                                    _model.chatHistory =
                                        functions.saveChatHistory(
                                            _model.chatHistory,
                                            functions.convertToJSON(
                                                _model.textController.text));
                                  });
                                  _model.chatGPTResponse =
                                      await OpenAIChatGPTGroup
                                          .sendFullPromptCall
                                          .call(
                                    apiKey:
                                        'sk-xxxxxxxxxx',
                                    promptJson: _model.chatHistory,
                                  );

Here’s saveChatHistory() function:

dynamic saveChatHistory(
  dynamic chatHistory,
  dynamic newChat,
) {
  // If chatHistory isn't a list, make it a list and then add newChat
  if (chatHistory is List) {
    chatHistory.add(newChat);
    return chatHistory;
  } else {
    return [newChat];
  }
}

Here’s convertToJSON() function:

dynamic convertToJSON(String prompt) {
  // take the prompt and return a JSON with form [{"role": "user", "content": prompt}]
  return json.decode('{"role": "user", "content": "$prompt"}');
}

I’ve tried adding the "system" message in the convertToJSON() function like this:

dynamic convertToJSON(String prompt) {
  // take the prompt and return a JSON with form [{"role": "user", "content": prompt}]
  return json.decode('[{"role": "system", "content": "system message"}, {"role": "user", "content": "$prompt"}]');
}

but this is returning a 400 error code that indicates bad request.