var client = new ManagementClient(
    new CertificateCloudCredentials(subscriptionId, certificate));
//...
var requestMessage = new HttpRequestMessage(HttpMethod.Get, apiURL);
await client.Credentials.ProcessHttpRequestAsync(requestMessage,
    CancellationToken.None);
var httpClient = new HttpClient();
HttpResponseMessage response = await httpClient.SendAsync(requestMessage);
// TODO: process response, maybe:
var responseText = response.AsString();