Change Azure website app settings from code

You can also use the Azure Fluent Api.

using Microsoft.Azure.Management.Fluent;
using Microsoft.Azure.Management.ResourceManager.Fluent;
using Microsoft.Azure.Management.ResourceManager.Fluent.Authentication;
using Microsoft.Azure.Management.ResourceManager.Fluent.Core;

…

public void UpdateSetting(string key, string value)
{
    string tenantId = "a5fd91ad-....-....-....-............";
    string clientSecret = "8a9mSPas....................................=";
    string clientId = "3030efa6-....-....-....-............";
    string subscriptionId = "a4a5aff6-....-....-....-............";

    var azureCredentials = new AzureCredentials(new
      ServicePrincipalLoginInformation
    {
        ClientId = clientId,
        ClientSecret = clientSecret
    }, tenantId, AzureEnvironment.AzureGlobalCloud);

    var _azure = Azure
   .Configure()
   .WithLogLevel(HttpLoggingDelegatingHandler.Level.Basic)
   .Authenticate(azureCredentials)
   .WithSubscription(subscriptionId);

    var appResourceId = "/subscriptions/xxx/resourceGroups/xxx/providers/Microsoft.Web/sites/xxx"; //Get From WebApp -> Properties -> Resource ID

    var webapp = _azure.WebApps.GetById(appResourceId);

    //Set App Setting Key and Value
    webapp.Update()
        .WithAppSetting(key, value)
        .Apply();
}