csharp

public class EditorSendingContentNotificationHandler : INotificationHandler<SendingContentNotification>
{
    public void Handle(SendingContentNotification notification)
    {
        // something to determine if this is content you want to modify...
        if (notification.Content.ContentTypeAlias.Equals("blogpost"))
        {
            foreach (var variant in notification.Content.Variants)
            {
                var shouldBeManadatory = variant.Tabs.SelectMany(f => f.Properties)
                    .FirstOrDefault(f => f.Alias.InvariantEquals("propertyAlias"));
                if (shouldBeManadatory is not null)
                {
                    shouldBeManadatory.Validation.Mandatory = true;
                }
            }
        }
    }
}