Demonstrates how to map a domain name to an Azure website programmatically

MapAppServiceDomain.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using Microsoft.IdentityModel.Clients.ActiveDirectory;
using Newtonsoft.Json.Linq;

static class DomainMapper
{
  public static async Task MapDomainName(string domainName)
  {
    string clientId = "your-azure-ad-app-client-id"; // create an app in azure ad and obtain these numbers their
    string clientSecret = "your-azure-ad-app-client-secret";
    string tenantId = "your-tenant-id"; // e.g. myazuredomain.com
    string resourceGroup = "your-resource-group"; // the resource group the website belongs to
    string subscriptionId = "your-azure-subscription-id"; // the subscription id the website belongs to
    string websiteName ="your-website-name"; // the name of your website

    ClientCredential clientCredential = new ClientCredential(clientId, clientSecret);
    AuthenticationContext context = new AuthenticationContext($"https://login.windows.net/{tenantId}");
    AuthenticationResult authenticationResult = await context.AcquireTokenAsync("https://management.azure.com/", clientCredential);
    if (authenticationResult == null)
    {
      // handle the auth error
      return;
    }
    string accessToken = authenticationResult?.AccessToken;

    HttpClient client = new HttpClient();
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", accessToken);
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

    IReadOnlyCollection<string> hostnames = await GetExistingHostnames(subscriptionId, resourceGroup, websiteName, client);
    if (!hostnames.Contains(command.DomainName))
    {
      string domain = Uri.EscapeUriString(command.DomainName);
      string updateUri = $"https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroup}/providers/Microsoft.Web/sites/{websiteName}/hostNameBindings/{domain}?api-version=2016-08-01";
      HttpResponseMessage response = await client.PutAsync(updateUri, null);
      response.EnsureSuccessStatusCode();
    }
  }
  
  private static async Task<IReadOnlyCollection<string>> GetExistingHostnames(string subscriptionId, string resourceGroup, string websiteName,
            HttpClient client)
  {
    string getConfigurationUri =
        $"https://management.azure.com/subscriptions/{subscriptionId}/resourceGroups/{resourceGroup}/providers/Microsoft.Web/sites/{websiteName}?api-version=2015-04-01";
    string result = await client.GetStringAsync(getConfigurationUri);

    JObject json = JObject.Parse(result);
    JArray jsonHostnames = (JArray) (json["properties"]["hostNames"]);
    List<string> hostnames = jsonHostnames.Select(x => (string) x).ToList();
    return hostnames;
  }
}