open New Popup Window on ImageButton Click Event

Dim url As String = "popup_BankTransferRequest.aspx"
        Dim sb As New StringBuilder()
        sb.Append("<script type = 'text/javascript'>")
        sb.Append("window.open('")
        sb.Append(url)
        sb.Append("');")
        sb.Append("</script>")
        ClientScript.RegisterStartupScript(Me.GetType(), _"script", sb.ToString())

Hello Guyz, I am using this code for opening a new window as popup, Window is opening but not as a popup. it is opening as a new TAB.

Please Help Solve this Issue.