(AzureCXP) Add Note regarding X5T

Added a note describing how to properly generate the x5t value.

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cc: @rwike77
GitHub Issue: https://github.com/MicrosoftDocs/azure-docs/issues/40871

Author: Frank Hu

articles/active-directory/develop/active-directory-certificate-credentials.md

@@ -129,4 +129,7 @@ In the Azure app registration for the client application:
 
 ## Code sample
 
+> [!NOTE]
+> You must calculate the X5T header by using the certificate's hash and  converting it to a base64 string. In C# it would look something similar to that of : `System.Convert.ToBase64String(cert.GetCertHash());`
+
 The code sample on [Authenticating to Azure AD in daemon apps with certificates](https://github.com/Azure-Samples/active-directory-dotnet-daemon-certificate-credential) shows how an application uses its own credentials for authentication. It also shows how you can [create a self-signed certificate](https://github.com/Azure-Samples/active-directory-dotnet-daemon-certificate-credential#create-a-self-signed-certificate) using the `New-SelfSignedCertificate` Powershell command. You can also take advantage and use the [app creation scripts](https://github.com/Azure-Samples/active-directory-dotnet-daemon-certificate-credential/blob/master/AppCreationScripts/AppCreationScripts.md) to create the certificates, compute the thumbprint, and so on.