Twitterizer - How to post a reply?

Question

I'm using the Twitterizer wrapper for a new ASP.NET project. I have a gridview showing me the twitter timeline. I have a button in every row to reply to a specific status message. But i just reply a message like this: "@screenName this is a reply". This is just a new statusupdate on my twitter. But the in_reply_to_user_id field of that status is not filled in. What should i do to reply correctly to a message?