Add javascript alert to asp.net button

Method: 1

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 <script> 

function alert_me() {

            return confirm('Hello!');

        }

 </script>

 <asp:Button ID="Button4"  OnClientClick="alert_me()"

                Text="Click Me" runat="server" />

            <br />

Method: 2

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<asp:Button ID="Button5"  OnClientClick="return confirm('are you sure?')"
                Text="Click Me" runat="server" />