Hi! Thank you for this solution! I follow each steps correctly and I try to log something on a text file. At the instruction:

await httpClientLead.GetStringAsync(formUrl)

log the error:

System.Net.Http.HttpRequestException: Il codice di stato della risposta non indica la riuscita dell'operazione: 400 (Bad Request).
in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
in FacebookLeadAdsWebhooks.Controller.WebhooksController.d__1.MoveNext() in C:\Users\xxxxx\Downloads\fb\Facebook-ASP.NET-Webhook-master\src\FacebookLeadAdsWebhooks\FacebookLeadAdsWebhooks\Controller\WebhooksController.cs:riga 74

Why?