How to structure a querystring in a postbackurl

 I would like to have a series of buttons that postback to the original page but with a dynamic querystring. Something like this:
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click Button1.PostBackUrl = "StructureAmendment.aspx?TIPID=" + TextBox1.Text.Trim End Sub But the above does not work until the second click. Can someone tell me how to do this?
Thanks
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