API call (httpclient / IHttpClientFactory ) with API_Key Authorization

Question

I want to call third party API with API_Key Authorization. Please find my code and it return "StatusCode: 401, ReasonPhrase: 'Unauthorized', Version: 1.1"
I tried to add API Key by 2 approach and both gives above response. Please advise.

var data = new StringContent(JsonConvert.SerializeObject(DocumentRequest), Encoding.UTF8, "application/json");
var httpClientHandler = new HttpClientHandler();
httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, sslPolicyErrors) =>
{
    return true;
};

    using (var client = new HttpClient(httpClientHandler))
{
    client.DefaultRequestHeaders.Accept.Clear();

    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));

    //client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(apiKey, apiKeyValue);
    
    client.DefaultRequestHeaders.Add(apiKey, apiKeyValue);
        
    var response = await client
    .PostAsync(apiEndPoint, data, cancellationToken)
    .ConfigureAwait(false);
    
    var errorResponse = response.Content.ReadAsStringAsync().Result;
    
    if (!response.IsSuccessStatusCode)
    {
        _logger.LogError("Request rejected by validation process " + errorResponse);
        throw new Exception($"{nameof(response.StatusCode)}:{response.StatusCode}:{errorResponse}, Request rejected.");
    }
    var responseBody = await response.Content.ReadAsStringAsync().ConfigureAwait(false);

    return responseBody;
}