Using the Dart Generator for an api - how to set the headers

After downloading the generated client for Dart and setting up in my flutter application

  ApiClient client = ApiClient(basePath: "https://api.openai.com/v1");
  var api = OpenAIApi(client);

How do I set the authentication headers?

calling the following createCompletion, there is no place to set headerParams
api.createCompletion(body);

Future<CreateCompletionResponse> createCompletion(CreateCompletionRequest body) async {
    Object postBody = body;

    // verify required params are set
    if(body == null) {
     throw new ApiException(400, "Missing required param: body");
    }

    // create path and map variables
    String path = "/completions".replaceAll("{format}","json");

    // query params
    List<QueryParam> queryParams = [];
   Map<String, String> headerParams = {};  // <<<<<<<<<<<<<<<<<<< here you can see they are always empty
    Map<String, String> formParams = {};
    
    List<String> contentTypes = ["application/json"];

    String contentType = contentTypes.length > 0 ? contentTypes[0] : "application/json";
    List<String> authNames = [];

    if(contentType.startsWith("multipart/form-data")) {
      bool hasFields = false;
      MultipartRequest mp = new MultipartRequest(null, null);
      if(hasFields)
        postBody = mp;
    }
    else {
          }

    var response = await apiClient.invokeAPI(path,
                                             'POST',
                                             queryParams,
                                             postBody,
                                             headerParams,
                                             formParams,
                                             contentType,
                                             authNames);

    if(response.statusCode >= 400) {
      throw new ApiException(response.statusCode, response.body);
    } else if(response.body != null) {
      return
          apiClient.deserialize(response.body, 'CreateCompletionResponse') as CreateCompletionResponse ;
    } else {
      return null;
    }

Trace the generated Dart client from ApiClient and OpenAIApi.createCompletion to ApiClient.invokeAPI, focusing on headerParams and authNames. Reproduce the call with the shown OpenAI base path and verify that the authentication header reaches the request and that createCompletion still succeeds.