Probelms with Modal Popup via server in code behind

Hi all

I am having problems while adding the modal popup dynamically to my page on the server side on a (web user control). When I set the "TargetControlId" or the "PopupControlId" like this -

ModalPopupExtender modPop = new ModalPopupExtender();
TextBox tbMsg = new TextBox();
tbMsg.ID = "tbMsg";
tbMsg.Text = "Thanks!";
modPop.ID = "modulePopup";
modPop.TargetControlID = hiddenField.ID; //made this hidden field in the user control
modPop.PopupControlID = tbMsg.ClientID;
modPop.BackgroundCssClass = "modalBackground";
modPop.X = 200;
modPop.Y = 200;

Now what is happening is that the ID's that go to the modal popup javascript (ScriptResource etc) I see that it goes in as the name of the textbox itself "tbmsg" instead of the client ID ? so I see something like document.getElementByID("tbMsg") !!!

What am I supposed to do??

Thanks in advance

Sanchita

hiddenField.ClientID will get the ID which is generated by ASP.net