Pop up window on LINKBUTTON click

Hi

I have a link button and when clicked it should show a pop up window.

The problem is that the first time i click on the linkbutton, the pop up window does not show.  The pop up window only appears when i click on the linkbutton on the second time.  Why is this so?  At my linkbutton click event, i put the code:

Session("ProductName") = LinkButton1.Text
LinkButton1.Attributes.Add("onclick", "window.open('promo.aspx',null,'height=360, width=420,status= no, resizable= no, scrollbars=no, toolbar=no,location=no,menubar=no ');")

I tried the same method using hyperlink it works.  I only have to click the hyperlink ONCE then the pop up window would appear.  How should i do it for link button???  How should i change my code?