You can use the Behavior property of the ConfirmButtonExtender and add the javascript function to it, this way the javascript function will be called before the ConfirmText property of the ConfirmButtonExtender.
Modify your markup to the following:
<asp:Button ID="UpdatebuttonUpdaterID" runat="server" Text="Update" CssClass="create_role_button_in"
OnClick="UpdatebuttonUpdaterID_Click" OnClientClick="return OnClientClickUpdate();"/>
<ajaxToolkit:ConfirmButtonExtender BehaviorID="confirmBehavior" ID="ConfirmButtonExtender3" runat="server" TargetControlID="UpdatebuttonUpdaterID"
ConfirmText="Do you want to send this Conversion Rate for Approval?" ConfirmOnFormSubmit="true" OnClientCancel="CancelClick"/>
<script type="text/ecmascript">
Sys.Application.add_load(wireEvents);
function wireEvents() {
var behavior = $find("confirmBehavior");
behavior.add_showing(OnClientClickUpdate);
}
function OnClientClickUpdate() {
alert("hello");
}
</script>
Hope it helps, source for this answer Here.