The button can only do one thing. If you tell it to change the browser location when you click, that's what it will do, even if it's a submit button.
This isn't a very ASP.NET way of doing what you are doing.
You'd probably be better to use a regular asp button, do what you need when the form submits (in the button click handler) and then use Response.Redirect() to send the user to the new page.
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Peter