Make an alert box in asp .net version 2.0 appear before page

Make an alert box in asp .net version 2.0 appear before page load , refresh
in the Page_Load add

Me.Button.Attributes.Add("onClick", "alert('You Message')")

And after it has been clicked once, and u dont want to see it again
Me.Button.Attributes.Remove("onClick")

You can put a condition on that, to check if its clicked or not